array_search()

(PHP 4 >= 4.0.5, PHP 5, PHP 7)

在数组中搜索给定的值，如果成功则返回首个相应的键名

说明

array_search(mixed $needle,array $haystack[,bool $strict= false]): mixed

大海捞针，在大海（$haystack）中搜索针（$needle参数）。

参数

$needle

搜索的值。

Note:

如果$needle是字符串，则比较以区分大小写的方式进行。

$haystack

这个数组。

$strict

如果可选的第三个参数$strict为TRUE，则array_search()将在$haystack中检查完全相同的元素。这意味着同样严格比较$haystack里$needle的类型，并且对象需是同一个实例。

返回值

如果找到了$needle则返回它的键，否则返回FALSE。

如果$needle在$haystack中出现不止一次，则返回第一个匹配的键。要返回所有匹配值的键，应该用array_keys()加上可选参数$search_value来代替。

Warning

此函数可能返回布尔值FALSE，但也可能返回等同于FALSE的非布尔值。请阅读布尔类型章节以获取更多信息。应使用===运算符来测试此函数的返回值。

更新日志

范例

Example #1array_search()例子

<?php
$array = array(0 => 'blue', 1 => 'red', 2 => 'green', 3 => 'red');
$key = array_search('green', $array); // $key = 2;
$key = array_search('red', $array);   // $key = 1;
?>

参见

• array_keys() 返回数组中部分的或所有的键名
• array_values() 返回数组中所有的值
• array_key_exists() 检查数组里是否有指定的键名或索引
• in_array() 检查数组中是否存在某个值

in (PHP 5 >= 5.5.0) you don't have to write your own function to search through a multi dimensional array
ex : 
$userdb=Array
(
  (0) => Array
    (
      (uid) => '100',
      (name) => 'Sandra Shush',
      (url) => 'urlof100'
    ),
  (1) => Array
    (
      (uid) => '5465',
      (name) => 'Stefanie Mcmohn',
      (pic_square) => 'urlof100'
    ),
  (2) => Array
    (
      (uid) => '40489',
      (name) => 'Michael',
      (pic_square) => 'urlof40489'
    )
);
simply u can use this
$key = array_search(40489, array_column($userdb, 'uid'));

About searcing in multi-dimentional arrays; two notes on "xfoxawy at gmail dot com";
It perfectly searches through multi-dimentional arrays combined with array_column() (min php 5.5.0) but it may not return the values you'd expect.
<?php array_search($needle, array_column($array, 'key')); ?>
Since array_column() will produce a resulting array; it won't preserve your multi-dimentional array's keys. So if you check against your keys, it will fail.
For example;
<?php
$people = array(
 2 => array(
  'name' => 'John',
  'fav_color' => 'green'
 ),
 5=> array(
  'name' => 'Samuel',
  'fav_color' => 'blue'
 )
);
$found_key = array_search('blue', array_column($people, 'fav_color'));
?>
Here, you could expect that the $found_key would be "5" but it's NOT. It will be 1. Since it's the second element of the produced array by the array_column() function.
Secondly, if your array is big, I would recommend you to first assign a new variable so that it wouldn't call array_column() for each element it searches. For a better performance, you could do;
<?php
$colors = array_column($people, 'fav_color');
$found_key = array_search('blue', $colors);
?>

the recursive function by tony have a small bug. it failes when a key is 0
here is the corrected version of this helpful function:
<?php
function recursive_array_search($needle,$haystack) {
  foreach($haystack as $key=>$value) {
    $current_key=$key;
    if($needle===$value OR (is_array($value) && recursive_array_search($needle,$value) !== false)) {
      return $current_key;
    }
  }
  return false;
}
?>

If you are using the result of array_search in a condition statement, make sure you use the === operator instead of == to test whether or not it found a match. Otherwise, searching through an array with numeric indicies will result in index 0 always getting evaluated as false/null. This nuance cost me a lot of time and sanity, so I hope this helps someone. In case you don't know what I'm talking about, here's an example:
<?php
$code = array("a", "b", "a", "c", "a", "b", "b"); // infamous abacabb mortal kombat code :-P
// this is WRONG
while (($key = array_search("a", $code)) != NULL)
{
 // infinite loop, regardless of the unset
 unset($code[$key]);
}
// this is _RIGHT_
while (($key = array_search("a", $code)) !== NULL)
{
 // loop will terminate
 unset($code[$key]);
}
?>

$array = ['a', 'b', 'c'];
$key = array_search('a', $array); //$key = 0
if ($key) 
{
//even a element is found in array, but if (0) means false
//...
}
//the correct way
if (false !== $key)
{
//....
}
It's what the document stated "may also return a non-Boolean value which evaluates to FALSE."

About searcing in multi-dimentional arrays; 
note on "xfoxawy at gmail dot com" and turabgarip at gmail dot com;
$xx = array_column($array, 'NAME', 'ID');
will produce an array like :
 $xx = [
[ID_val] => NAME_val
[ID_val] => NAME_val
    ]
so: 
$yy = array_search('tesxt', array_column($array, 'NAME', 'ID'));
will output expected ID;

/*
Be careful!!!
when i in php7.1.5
below program return int 0
is it a bug?
*/
array_search(0,['a','b','c'])

Simple way to get variable name by using array_search function:
<?php
function varname($var){
  return (isset($var))? array_search($var, $GLOBALS) : false;
}
$boogie = 'tonight';
echo varname($boogie);
?>

for searching case insensitive better this:
<?php
array_search(strtolower($element),array_map('strtolower',$array));
?>

hey i have a easy multidimensional array search function 
<?php
function search($array, $key, $value)
{
  $results = array();
  if (is_array($array))
  {
    if (isset($array[$key]) && $array[$key] == $value)
      $results[] = $array;
    foreach ($array as $subarray)
      $results = array_merge($results, search($subarray, $key, $value));
  }
  return $results;
}
?>

I was going to complain bitterly about array_search() using zero-based indexes, but then I realized I should be using in_array() instead.
// if ( isset( $_GET['table'] ) and array_search( $_GET['table'], $valid_tables) ) { // BAD: fails on first[0] element
// if ( isset( $_GET['table'] ) and ( FALSE !== array_search( $_GET['table'], $valid_tables) ) ) { OK: but wasteful and convoluted
  if ( isset( $_GET['table'] ) and in_array( $_GET['table'], $valid_tables) ) { // BETTER
The essence is this: if you really want to know the location of an element in an array, then use array_search, else if you only want to know whether that element exists, then use in_array()

Be careful when search for indexes from array_keys() if you have a mixed associative array it will return both strings and integers resulting in comparison errors
<?php
// This syntax is allowed by PHP
$arr = array(
 "nice",
 "car" => "fast",
 "none"
);
// Print the keys
var_dump(array_keys($arr));
/* The above prints this, as you can see we have mixed keys
array(3) {
 [0]=>
 int(0)
 [1]=>
 string(3) "car"
 [2]=>
 int(1)
}
*/
// Search for the index "car"
var_dump(array_search("car", array_keys($arr))); // Prints "int(0)" which is WRONG
?>
This happens because PHP, when comparing strings and integers, casts strings TO integers and this results in most of the cases in string becoming 0, so that's why when array_search() compares the first index (0) with the key "car" it gets true because apparently ("car" == 0) IS TRUE.
Setting array_search() to strict mode won't solve the problem because then array_search("0", array_keys($arr)) would return false even if an element with index 0 exists.
So my solution just converts all indexes from array_keys() to strings and then compares them correctly:
<?php
var_dump(array_search("car", array_map("strval", array_keys($arr)))); // Prints "int(1)" which is CORRECT
?>

Noted some interesting behaviour when using array_search to find the correct index in an array containing an index with a value of 0.
The following works as expected.
<?php
$dataSet = [];
$dataSet[] = ['name' => 'Row A', 'items' => ['ItemA' => 1, 'ItemB' => 2]];
$dataSet[] = ['name' => 'Row B', 'items' => ['ItemA' => 1, 'ItemB' => 2]];
$rows = [];
foreach ($dataSet as $dataRow) {
  $row = [
    $dataRow['name'],
    'ItemA',
    'ItemB'
  ];
  
  foreach ($dataRow['items'] as $key => $value) {
    $index = array_search($key, $row);
    $row[$index] = $value;
  }
  
  $rows[] = $row;
}
print_r($rows);
It returns:
Array
(
  [0] => Array
    (
      [0] => Row A
      [1] => 1
      [2] => 2
    )
  [1] => Array
    (
      [0] => Row B
      [1] => 1
      [2] => 2
    )
)
However, the following seems to totally ignore the index containing the value 0 and thus returns the wrong index.
<?php
$dataSet = [];
$dataSet[] = ['name' => 'Row A', 'items' => ['ItemA' => 1, 'ItemB' => 2]];
$dataSet[] = ['name' => 'Row B', 'items' => ['ItemA' => 0, 'ItemB' => 2]];
$rows = [];
foreach ($dataSet as $dataRow) {
  $row = [
    $dataRow['name'],
    'ItemA',
    'ItemB'
  ];
  
  foreach ($dataRow['items'] as $key => $value) {
    $index = array_search($key, $row);
    $row[$index] = $value;
  }
  
  $rows[] = $row;
}
print_r($rows);
It returns:
Array
(
  [0] => Array
    (
      [0] => Row A
      [1] => 1
      [2] => 2
    )
  [1] => Array
    (
      [0] => Row B
      [1] => 2
      [2] => ItemB
    )
)
However, setting ItemA's value on Row B to '' or false works fine. Also, passing in the third optional parameter [, bool $strict = true ] works correctly as well. Is this intentional behaviour?

To expand on previous comments, here are some examples of
where using array_search within an IF statement can go
wrong when you want to use the array key thats returned.
Take the following two arrays you wish to search:
<?php
$fruit_array = array("apple", "pear", "orange");
$fruit_array = array("a" => "apple", "b" => "pear", "c" => "orange");
if ($i = array_search("apple", $fruit_array))
//PROBLEM: the first array returns a key of 0 and IF treats it as FALSE
if (is_numeric($i = array_search("apple", $fruit_array)))
//PROBLEM: works on numeric keys of the first array but fails on the second
if ($i = is_numeric(array_search("apple", $fruit_array)))
//PROBLEM: using the above in the wrong order causes $i to always equal 1
if ($i = array_search("apple", $fruit_array) !== FALSE)
//PROBLEM: explicit with no extra brackets causes $i to always equal 1
if (($i = array_search("apple", $fruit_array)) !== FALSE)
//YES: works on both arrays returning their keys
?>

Expanding on the comment by hansen{}cointel.de:
When searching for a string and the array contains 0 (zero), the string is casted to (int) by the type-casting which is always 0 (perhaps the opposite is the proper behaviour, the array value 0 should have been casted to string). That produces unexpected results if strict comparison is not used:
<?php
$a = array(0, "str1", "str2", "str3");
echo "
str1 = ".array_search("str1", $a).",
str2 = ".array_search("str2", $a).",
str3 = ".array_search("str3", $a).",
str1 strict = ".array_search("str1", $a, true).",
str2 strict = ".array_search("str2", $a, true).",
str3 strict = ".array_search("str3", $a, true);
?>
This will return:
str1 = 0, str2 = 0, str3 = 0, str1 strict = 1, str2 strict = 2, str3 strict = 3

Combining syntax of array_search() and functionality of array_keys() to get all key=>value associations of an array with the given search-value:
<?php
function array_search_values( $m_needle, $a_haystack, $b_strict = false){
  return array_intersect_key( $a_haystack, array_flip( array_keys( $a_haystack, $m_needle, $b_strict)));
}
?>
Usage:
<?php
$array1 = array( 'pre'=>'2', 1, 2, 3, '1', '2', '3', 'post'=>2);
print_r( array_search_values( '2', $array1));
print_r( array_search_values( '2', $array1, true));
print_r( array_search_values( 2, $array1, true));
?>
Will return:
array(4) {
  ["pre"] =>
  string(1) "2"
  [1] =>
  int(2)
  [4] =>
  string(1) "2"
  ["post"] =>
  int(2)
}
array(2) {
  ["pre"] =>
  string(1) "2"
  [4] =>
  string(1) "2"
}
array(2) {
  [1] =>
  int(2)
  ["post"] =>
  int(2)
}

FYI, remember that strict mode is something that might save you hours.
If you're searching for a string and you have a "true" boolean on the way - you will get it as result (first occurrence). Example below:
<?php
$arr = [
  'foo'  => 'bar',
  'abc'  => 'def',
  'bool'  => true,
  'target' => 'xyz'
];
var_dump( array_search( 'xyz', $arr ) ); //bool
var_dump( array_search( 'xyz', $arr, true ) ); //target
?>

Better solution of multidimensional searching.
<?php
function multidimensional_search($parents, $searched) {
 if (empty($searched) || empty($parents)) {
  return false;
 }
 
 foreach ($parents as $key => $value) {
  $exists = true;
  foreach ($searched as $skey => $svalue) {
   $exists = ($exists && IsSet($parents[$key][$skey]) && $parents[$key][$skey] == $svalue);
  }
  if($exists){ return $key; }
 }
 
 return false;
}
$parents = array();
$parents[] = array('date'=>1320883200, 'uid'=>3);
$parents[] = array('date'=>1320883200, 'uid'=>5);
$parents[] = array('date'=>1318204800, 'uid'=>5);
echo multidimensional_search($parents, array('date'=>1320883200, 'uid'=>5)); // 1
?>

I needed a way to return the value of a single specific key, thus:
<?php
function recursive_return_array_value_by_key($needle, $haystack){
  $return = false;
  foreach($haystack as $key => $val){
    if(is_array($val)){
      $return = recursive_return_array_value_by_key($needle, $val);
    }
    else if($needle === $key){
      return "$val\n";
    }
  }
  return $return;
}
?>

If you only know a part of a value in an array and want to know the complete value, you can use the following function:
<?php
function array_find($needle, $haystack)
{
  foreach ($haystack as $item)
  {
   if (strpos($item, $needle) !== FALSE)
   {
     return $item;
     break;
   }
  }
}
?>
The function returns the complete first value of $haystack that contains $needle.

I had an array of arrays and needed to find the key of an element by comparing actual reference.
Beware that even with strict equality (===) php will equate arrays via their elements recursively, not by a simple internal pointer check as with class objects. The === can be slow for massive arrays and also crash if they contain circular references.
This function performs reference sniffing in order to return the key for an element that is exactly a reference of needle.
<?php 
function array_ref_search(&$v, array &$s)
{
  if(is_object($v)){ return array_search($v, $s, true); }
  foreach($s as $rK => &$rV)
  { // reference sniff
    $tV = $v;
    if( ($rV === ($v = 1)) && ($rV === ($v = 0)) ){
    $v = $tV; return $rK; }
    $v = $tV;
  }
  return false; // use null for php < 4.2.0
}
$list  = array();
$list['A'] = &$valA; $list['B'] = &$valB;
$valA = 1; $valB = 1;
echo 'array_ref_search: ', array_ref_search($valB, $list), '</br>'; // key 'B'
echo 'array_search:   ', array_search($valB, $list, true), '</br>'; // key 'A'
$valA = array(1,2,3); $valB = array(1,2,3);
echo 'array_ref_search: ', array_ref_search($valB, $list), '</br>'; // key 'B'
echo 'array_search:   ', array_search($valB, $list, true), '</br>'; // key 'A' because ($valA === $valB) is true by elements
$valB[] = &$valB; // circular reference
echo 'array_ref_search: ', array_ref_search($valB, $list), '</br>'; // key 'B'
echo 'array_search:   ', array_search($valB, $list, true), '</br>'; // crash because ($valB === $valB) causes infinite loop
?>

I was trying to use array_search to retrieve all the values that match a given needle, but it turns out only the first match key is returned. I built this little function, which works just like array_search, but returns all the keys that match a given needle instead. The output is an array.
<?php
$haystack = array('a','b','a','b');
$needle = 'a';
print_r(array_search_all($needle, $haystack));
//Output will be
// Array
// (
//     [0]=>1
//     [1]=>3
// )
function array_search_all($needle, $haystack)
{#array_search_match($needle, $haystack) returns all the keys of the values that match $needle in $haystack
  foreach ($haystack as $k=>$v) {
  
    if($haystack[$k]==$needle){
    
      $array[] = $k;
    }
  }
  return ($array);
  
}
?>

one thing to be very aware of is that array_search() will fail if the needle is a string and the array itself contains values that are mixture of numbers and strings. (or even a string that looks like a number)
The problem is that unless you specify "strict" the match is done using ==  and in that case any string will match a numeric value of zero which is not what you want.
-----
also, php can lookup an index pretty darn fast. for many scenarios, it is practical to maintain multiple arrays, one in which the index of the array is the search key and the normal array that contains the data.
<?php
 $normal[$index] = array('key'=>$key, 'data'=>'foo');
 $inverse[$key] = $index;
 //very fast lookup, this beats any other kind of search
 if (array_key_exists($key, $inverse)) 
 {
  $index = $inverse[$key];
  return $normal[$index];
 }
?>

A simple recursive array_search function : 
<?php
function recursive_array_search($needle,$haystack) {
  foreach($haystack as $key=>$value) {
    $current_key=$key;
    if($needle===$value OR (is_array($value) && recursive_array_search($needle,$value))) {
      return $current_key;
    }
  }
  return false;
}
?>

A better array_isearch would be to store all results in an array, then return the KEYS stored in $found, such as:
<?php
function array_isearch($str, $array){
 $found = array();
 foreach ($array as $k => $v)
   if (strtolower($v) == strtolower($str)) $found[] = $k;
 return $found;
}
?>
To use, simply have an array to search from then search it, for example:
<?php
function array_isearch($str, $array) {
 $found = array();
 foreach($array as $k => $v)
  if(strtolower($v) == strtolower($str)) $found[] = $k;
 return $found;
}
$stored = "these are an array";
$stored = explode(" ", $stored);
$compare = array("these", "are", "some", "results", "stored", "in", "an", "array");
foreach($stored as $store) {
 $results = array_isearch($store, $compare);
 foreach($results as $key => $result)
  echo "Key: ".$results[$key]."<br />Found: ".$compare[$result]."<br />";
}
?>
Hope this helps :-)
-Rob

For data lookups that are fast in both directions (key to value, value to key), consider storing and updating both the array and its array_flip() version.

The fastest way to search for a cached value, to find if there is a cycle in your data, to see if a given program state has occurred before, to see whether a value is in a set and then adding it, and for so many similar tasks, is just to do an array dereference:
  $Key=$SomeStateString1.'|'.$SomeStateString2; // State can have multiple substates
  if (@$StateSet[$Key]) // or use isset()
    ; // The key already exists: do optional processing here
  else
    ; // The key does not yet exist: do optional processing here
  $StateSet[$Key]=1; // Store state existence (or store associated data values)

you need to be careful if you are using array_search() with unset() for deleting a value in the searched indexed
// wrong way
$checked = ["1", "2", "3"]; // array with numbers in double quotes
unset($checked[array_search('on', $checked)]); // this deletes value at 0 index
// array_search returns mixed data type, which means any type of data will be returned
// in the above case it returned false and false is type casted to 0 
// so it deletes the value at index zero
the point to say is never use array_search() with unset() while deleting a value in an array using index.

this is for searching a value inside a multidimontionnal array, and then return the parent of the parent array that holds the value.
function recursive_array_search($needle,$haystack) {
  foreach($haystack as $key=>$value) {
    $current_key=$key;
    if($needle===$value OR (is_array($value) && recursive_array_search($needle,$value))) {
      return $haystack[$key];
    }
  }
  return false;
}

For multiarray values i got this solution ;)
$taxonomy = array(
  array('index' => array('AU200_AUD','CN50_USD')),
  array('mt' => array('XAG_GBP','XAG_HKD','XAU_GBP','XAU_HKD','XAU_JPY')),
  array('aud' => array('AUD_CAD','AUD_CHF','AUD_HKD','AUD_JPY','AUD_NZD','AUD_SGD','AUD_USD')),
  array('cad' => array('CAD_CHF','CAD_HKD','CAD_JPY','CAD_SGD','CHF_HKD','CHF_JPY','CHF_ZAR')),
  array('eur' => array('EUR_AUD','EUR_CAD','EUR_CHF','EUR_NOK','EUR_NZD','EUR_PLN','EUR_ZAR')),
  array('gbp' => array('GBP_AUD','GBP_CAD','GBP_CHF','GBP_USD','GBP_ZAR')),
  array('hkd' => array('HKD_JPY')),
  array('nzd' => array('NZD_CAD','NZD_CHF','NZD_HKD','NZD_JPY','NZD_SGD','NZD_USD')),
  array('sgd' => array('SGD_CHF','SGD_HKD','SGD_JPY')),
  array('try' => array('TRY_JPY')),
  array('usd' => array('USD_CAD','USD_CHF','USD_CNH','USD_SGD','USD_THB','USD_TRY','USD_ZAR')),
  array('zar' => array('ZAR_JPY')),
 );
 foreach ($taxonomies as $key => $tax) {
  foreach ($tax as $value) {
   if(array_search('SGD_HKD', $value)) $taxonomy = key($tax);
  }
 }
echo($taxonomy); // this print "sgd"

Despite PHP's amazing assortment of array functions and juggling maneuvers, I found myself needing a way to get the FULL array key mapping to a specific value. This function does that, and returns an array of the appropriate keys to get to said (first) value occurrence.
function array_recursive_search_key_map($needle, $haystack) {
  foreach($haystack as $first_level_key=>$value) {
    if ($needle === $value) {
      return array($first_level_key);
    } elseif (is_array($value)) {
      $callback = array_recursive_search_key_map($needle, $value);
      if ($callback) {
        return array_merge(array($first_level_key), $callback);
      }
    }
  }
  return false;
}
usage example:
-------------------
$nested_array = $sample_array = array(
  'a' => array(
    'one' => array ('aaa' => 'apple', 'bbb' => 'berry', 'ccc' => 'cantalope'),
    'two' => array ('ddd' => 'dog', 'eee' => 'elephant', 'fff' => 'fox')
  ),
  'b' => array(
    'three' => array ('ggg' => 'glad', 'hhh' => 'happy', 'iii' => 'insane'),
    'four' => array ('jjj' => 'jim', 'kkk' => 'kim', 'lll' => 'liam')
  ),
  'c' => array(
    'five' => array ('mmm' => 'mow', 'nnn' => 'no', 'ooo' => 'ohh'),
    'six' => array ('ppp' => 'pidgeon', 'qqq' => 'quail', 'rrr' => 'rooster')
  )
);
$search_value = 'insane';
$array_keymap = array_recursive_search_key_map($search_value, $nested_array);
var_dump($array_keymap);
// Outputs:
// array(3) {
// [0]=>
// string(1) "b"
// [1]=>
// string(5) "three"
// [2]=>
// string(3) "iii"
//}
----------------------------------------------
But again, with the above solution, PHP again falls short on how to dynamically access a specific element's value within the nested array. For that, I wrote a 2nd function to pull the value that was mapped above.
function array_get_nested_value($keymap, $array)
{
  $nest_depth = sizeof($keymap);
  $value = $array;
  for ($i = 0; $i < $nest_depth; $i++) {
    $value = $value[$keymap[$i]];
  }
  return $value;
}
usage example:
-------------------
echo array_get_nested_value($array_keymap, $nested_array);  // insane

Mohamed Nabil 
hallo every body This function matches two arrays like 
search an array like another or not array_match which can match 
function array_match($arr1,$arr2,$highlevel=false)
{
  $keys1=array_keys($arr1);
  $keys2=array_keys($arr2);
  $values1=array_values($arr1);
  $values2=array_values($arr2);
  
   if ($highlevel)
   {
   
   if ($keys1 === $keys2 && $values1 === $values2)
   {
     
     return true;
   }
    return false;
   }
   if ($keys1 == $keys2 && $values1 == $values2)
   {
     
     return true;
   }
    return false;
  
}
 var_dump(array_match(['id'=>'1','username'=>'2'],['id'=>'1','username'=>2]));
 //Output:true 
 var_dump(array_match(['id'=>'1','username'=>'2'],['id'=>'1','username'=>2],true));
 //Output:false

Example of a recursive binary search that returns the index rather than boolean.
<?php
// returns the index of needle in haystack
function binSearch($needle, $haystack)
{
  // n is only needed if counting depth of search
  global $n;
  $n++;
  // get the length of passed array
  $l = count($haystack);
  // if length is 0, problem
  if($l <= 0)
  {
    return -1;
  }
  // get the mid element
  $m = (($l+($l%2))/2);
  // if mid >= length (e.g. l=1)
  if($m >= $l)
  {
    $m = $m-1;
  }
  // get the indexed element to compare to the passed element and branch accordingly
  $compare = $haystack[$m];
  switch(true)
  {
    case($compare>$needle):
    {
      // recurse on the lower half
      $new_haystack = array_slice($haystack, 0, $m);
      $c = count($new_haystack);
      $r = binSearch($needle, $new_haystack);
      // return current index - (length of lower half - found index in lower half)
      return $m - ($c - $r);
      break;
    }
    case($compare<$needle):
    {
      // recurse on the upper half
      $new_haystack = array_slice($haystack, $m, ($l-$m));
      $c = count($new_haystack);
      $r = binSearch($needle, $new_haystack);
      // return current position + found index in upper half
      return $m + $r;
      break;
    }
    case($compare==$needle):
    {
      // found it, so return index
      return $m;
      break;
    }
  }
}
?>

There is no function to count the occurences of needle in haystack, so I made my own one...
<?php
function array_match($needle, $haystack)
{
  if( !is_array($haystack) ) return false;
  
  $i = 0;
  while( (in_array( $needle, $haystack )) != FALSE )
  {
    $i++;
    $haystack[array_search($needle, $haystack)] = md5($needle);
    reset($haystack);
  }
  
  return $i;
}
?>
I know it's a bit crappy, but don't ask me too much, I'm still only 13... ;)

Sometimes you need to find a given value in a sorted array or - if not found - detect the place where it should be. After that you can for example split the array into two halves, the greater and the smaller one.
greenmr, dennis.decoene and php at celerondude had all posted very good binary search functions but these functions all return false if the needle was not found in the haystack. I've tweaked greenmr's code a little:
<?php
function Array_BinarySearch( $needle, $haystack, $comparator , &$probe )
{
  $high = Count( $haystack ) -1;
  $low = 0;
  
  while ( $high >= $low )
  {
    $probe = Floor( ( $high + $low ) / 2 );
    $comparison = $comparator( $haystack[$probe], $needle );
    if ( $comparison < 0 )
    {
      $low = $probe +1;
    }
    elseif ( $comparison > 0 ) 
    {
      $high = $probe -1;
    }
    else
    {
      return true;
    }
  }
  //The loop ended without a match 
  //Compensate for needle greater than highest haystack element
  if($comparator($haystack[count($haystack)-1], $needle) < 0)
  {
    $probe = count($haystack);
  } 
  return false;
}
?>
Now, the function returns true if it finds something and false otherwise. If a needle was found, then $probe will contain it's position. Otherwise, $probe will contain position of where the needle would be if it were there :). This is possible because we pass $probe by reference.
Example:
<?php
//ultra-simple comparator :)
function CompareNumbers($obj, $needle)
{
  return $obj - $needle;
}
//use examples
$testArr = array(10, 20, 30, 40, 50);
$res = Array_BinarySearch(30, $testArr, 'CompareNumbers', $probe);
echo (int)$res.' '.$probe.'<br />';
//output is: 1 2 - found at position 2
$res = Array_BinarySearch(45, $testArr, 'CompareNumbers', $probe);
echo (int)$res.' '.$probe.'<br />';
//output is: 0 4 - not found, but it would be at position 4 (between 40 and 45)
 
$res = Array_BinarySearch(-3, $testArr, 'CompareNumbers', $probe);
echo (int)$res.' '.$probe.'<br />';
//output is: 0 0 - not found, but it would be at position 0 (before 10)
$res = Array_BinarySearch(300, $testArr, 'CompareNumbers', $probe);
echo (int)$res.' '.$probe.'<br />';
//output is: 0 5 - not found, but it would be at position 5 (after 50; note, that count($haystack) == 5)
?>
See original greenmr's note for additional details about usage of this binary search: http://php.net/manual/en/function.array-search.php#89413

Be aware of using array_search() in conditional statements.
$foo = array("one", "two");
if( $key = array_search("one", $foo) ){
 echo "Found";
}
else{
 echo "Not found";
}
//Ouput: Not found
"Not found" will be outputted beacause the returned key is 0 and that will be interpreted as false in conditional statements.

A variation of previous searches that returns an array of keys that match the given value:
<?php
function array_ksearch($array, $str)
{
  $result = array();
  for($i = 0; $i < count($array); next($array), $i++)
    if(strtolower(current($array)) == strtolower($str))
      array_push($result, key($array);
  
  return $result;
}
?>
Usage would be as follows:
<?php
$testArray = array('one' => 'test1', 'two' => 'test2', 'three' => 'test1', 'four' => 'test2', 'five' => 'test1');
  print_r(array_ksearch($testArray, 'test1'));
?>

Concerning how this function may return Boolean FALSE and the use of the === operator for testing the return value of this function, see this example:
$foundKey = array_search(12345, $myArray);
if(!isset($foundKey)){
  // If $myArray is null, then $foundKey will be null too.
  // Do something when both $myArray and $foundKey are null.
} elseif ($foundKey===false) {
  // $myArray is not null, but 12345 was not found in the $myArray array.
}else{
  // 12345 was found in the $myArray array.
}

I needed a way to find the parent hierarchy of a multidimensional array. Being the rogue that I am, I got to coding before searching the manual and came up with two little functions that will return a parent stack for a first find and a complete parent stack, similar in nature to the solution presented by jette at nerdgirl dot dk without all the extra stuff or use of eval(). ;)
<?php
/**
 * Gets the parent stack of a string array element if it is found within the 
 * parent array
 * 
 * This will not search objects within an array, though I suspect you could 
 * tweak it easily enough to do that
 *
 * @param string $child The string array element to search for
 * @param array $stack The stack to search within for the child
 * @return array An array containing the parent stack for the child if found,
 *        false otherwise
 */
function getParentStack($child, $stack) {
  foreach ($stack as $k => $v) {
    if (is_array($v)) {
      // If the current element of the array is an array, recurse it and capture the return
      $return = getParentStack($child, $v);
      
      // If the return is an array, stack it and return it
      if (is_array($return)) {
        return array($k => $return);
      }
    } else {
      // Since we are not on an array, compare directly
      if ($v == $child) {
        // And if we match, stack it and return it
        return array($k => $child);
      }
    }
  }
  
  // Return false since there was nothing found
  return false;
}
/**
 * Gets the complete parent stack of a string array element if it is found 
 * within the parent array
 * 
 * This will not search objects within an array, though I suspect you could 
 * tweak it easily enough to do that
 *
 * @param string $child The string array element to search for
 * @param array $stack The stack to search within for the child
 * @return array An array containing the parent stack for the child if found,
 *        false otherwise
 */
function getParentStackComplete($child, $stack) {
  $return = array();
  foreach ($stack as $k => $v) {
    if (is_array($v)) {
      // If the current element of the array is an array, recurse it 
      // and capture the return stack
      $stack = getParentStackComplete($child, $v);
      
      // If the return stack is an array, add it to the return
      if (is_array($stack) && !empty($stack)) {
        $return[$k] = $stack;
      }
    } else {
      // Since we are not on an array, compare directly
      if ($v == $child) {
        // And if we match, stack it and return it
        $return[$k] = $child;
      }
    }
  }
  
  // Return the stack
  return empty($return) ? false: $return;
}
// TESTING
$array = array(
  'balloon' => array(
    'red' => array(1 => 'Love', 'Valentine', 'Heart',),
    'green' => array(1 => 'Summertime', 'Hope',),
  ),
  'ribbon' => array(
    'yellow' => array(2 => 'Welcome',),
    'red' => array(3 => 'Love', 'Love',),
  ),
);
$s = getParentStack('Love', $array);
$c = getParentStackComplete('Love', $array);
var_dump($s, $c);
?>
Output:
array
 'balloon' => 
  array
   'red' => 
    array
     1 => string 'Love' (length=4)
array
 'balloon' => 
  array
   'red' => 
    array
     1 => string 'Love' (length=4)
 'ribbon' => 
  array
   'red' => 
    array
     3 => string 'Love' (length=4)
     4 => string 'Love' (length=4)

It has been said before: array_search is VERY slow. Everyone knows binary search is fast by design. Here is an implementation.
<?php
$arr=array(1,3,5,7,9,10,11,13);
$searchfor = 6;
echo binsearch($searchfor, $arr);
/**
 * @return integer
 * @param var $needle
 * @param array $haystack
 * @desc Feed a sorted array to $haystack and a value to search for to $needle.
       It will return false if not found or the index where it was found.
       This function is superfast. Try an array with 50.000 elements and search for something,
       you will be amazed.
*/
function binsearch($needle, $haystack)
{
  $high = count($haystack);
  $low = 0;
  
  while ($high - $low > 1){
    $probe = ($high + $low) / 2;
    if ($haystack[$probe] < $needle){
      $low = $probe;
    }else{
      $high = $probe;
    }
  }
  if ($high == count($haystack) || $haystack[$high] != $needle) {
    return false;
  }else {
    return $high;
  }
}
?>

Here is a version of binary search that is done via recursion instead of iteration. Remember that your data needs to be presorted!
<?php
static function Bin_Search(&$needle, &$haystack, $start, $end) {
    if($end < $start)
    {
      return false;
    }
    $mid = (int)(($end - $start) / 2) + $start;
 
    if($haystack[$mid] > $needle)
    {
      return Bin_Search($needle, $haystack, $start, $mid - 1);
    }
    else if($haystack[$mid] < $needle)
    {
      return Bin_Search($needle, $haystack, $mid + 1, $end);
    }
    else
    {
      return true;
    }
}
?>

You can remove some values from array, by using unset() and array_search().
<?php
$friends = array( 'Bob', 'Ann', 'Peter' ); // Two persons named 'Bob'
$find = 'Bob';
$key = array_search( $find, $friends ); // Find key of given value
if ($key != NULL || $key !== FALSE) {
  unset($friends[$key]); // remove key from array
}
// Now, $friends = array( 'Ann', 'Peter');
?>