array_column()

(PHP 5 >= 5.5.0, PHP 7)

返回数组中指定的一列

说明

array_column(array $input,mixed $column_key[,mixed $index_key= null]): array

array_column()返回$input数组中键值为$column_key的列，如果指定了可选参数$index_key，那么$input数组中的这一列的值将作为返回数组中对应值的键。

参数

$input: 需要取出数组列的多维数组。如果提供的是包含一组对象的数组，只有 public 属性会被直接取出。为了也能取出 private 和 protected 属性，类必须实现__get()和__isset()魔术方法。
$column_key: 需要返回值的列，它可以是索引数组的列索引，或者是关联数组的列的键，也可以是属性名。也可以是NULL，此时将返回整个数组（配合$index_key参数来重置数组键的时候，非常管用）
$index_key: 作为返回数组的索引/键的列，它可以是该列的整数索引，或者字符串键值。

返回值

从多维数组中返回单列数组。

更新日志

版本	说明
7.0.0	`$input`参数现在可以是包含对象的数组。

范例

从结果集中取出first names列

<?php
// Array representing a possible record set returned from a database
$records = array(
    array(
        'id' => 2135,
        'first_name' => 'John',
        'last_name' => 'Doe',
    ),
    array(
        'id' => 3245,
        'first_name' => 'Sally',
        'last_name' => 'Smith',
    ),
    array(
        'id' => 5342,
        'first_name' => 'Jane',
        'last_name' => 'Jones',
    ),
    array(
        'id' => 5623,
        'first_name' => 'Peter',
        'last_name' => 'Doe',
    )
);
 
$first_names = array_column($records, 'first_name');
print_r($first_names);
?>

以上例程会输出：

Array
(
    [0] => John
    [1] => Sally
    [2] => Jane
    [3] => Peter
)

从结果集中总取出last names列，用相应的id作为键值

<?php
// Using the $records array from Example #1
$last_names = array_column($records, 'last_name', 'id');
print_r($last_names);
?>

以上例程会输出：

Array
(
    [2135] => Doe
    [3245] => Smith
    [5342] => Jones
    [5623] => Doe
)

username 列是从对象获取 public 的"username"属性

<?php
class User
{
    public $username;
    public function __construct(string $username)
    {
        $this->username = $username;
    }
}
$users = [
    new User('user 1'),
    new User('user 2'),
    new User('user 3'),
];
print_r(array_column($users, 'username'));
?>

以上例程会输出：

Array
(
    [0] => user 1
    [1] => user 2
    [2] => user 3
)

获取 username 列，从对象通过魔术方法__get()获取 private 的"username"属性。

<?php
class Person
{
    private $name;
    public function __construct(string $name)
    {
        $this->name = $name;
    }
    public function __get($prop)
    {
        return $this->$prop;
    }
    public function __isset($prop) : bool
    {
        return isset($this->$prop);
    }
}
$people = [
    new Person('Fred'),
    new Person('Jane'),
    new Person('John'),
];
print_r(array_column($people, 'name'));
?>

以上例程会输出：

Array
(
    [0] => Fred
    [1] => Jane
    [2] => John
)

如果不提供__isset()，会返回空数组。

参见

» Recommended userland implementation for PHP lower than 5.5

if array_column does not exist the below solution will work.
if(!function_exists("array_column"))
{
  function array_column($array,$column_name)
  {
    return array_map(function($element) use($column_name){return $element[$column_name];}, $array);
  }
}

Because the function was not available in my version of PHP, I wrote my own version and extended it a little based on my needs.
When you give an $indexkey value of -1 it preserves the associated array key values.
EXAMPLE:
$sample = array(
  'test1' => array(
    'val1' = 10,
    'val2' = 100
  ),
  'test2' => array(
    'val1' = 20,
    'val2' = 200
  ),
  'test3' => array(
    'val1' = 30,
    'val2' = 300
  )
);
print_r(array_column_ext($sample,'val1'));
OUTPUT:
Array
(
  [0] => 10
  [1] => 20
  [2] => 30
)
print_r(array_column_ext($sample,'val1',-1));
OUTPUT:
Array
(
  ['test1'] => 10
  ['test2'] => 20
  ['test3'] => 30
)
print_r(array_column_ext($sample,'val1','val2'));
OUTPUT:
Array
(
  [100] => 10
  [200] => 20
  [300] => 30
)
<?php
function array_column_ext($array, $columnkey, $indexkey = null) {
  $result = array();
  foreach ($array as $subarray => $value) {
    if (array_key_exists($columnkey,$value)) { $val = $array[$subarray][$columnkey]; }
    else if ($columnkey === null) { $val = $value; }
    else { continue; }
      
    if ($indexkey === null) { $result[] = $val; }
    elseif ($indexkey == -1 || array_key_exists($indexkey,$value)) {
      $result[($indexkey == -1)?$subarray:$array[$subarray][$indexkey]] = $val;
    }
  }
  return $result;
}
?>

You can also use array_map fucntion if you haven't array_column().
example:
$a = array(
  array(
    'id' => 2135,
    'first_name' => 'John',
    'last_name' => 'Doe',
  ),
  array(
    'id' => 3245,
    'first_name' => 'Sally',
    'last_name' => 'Smith',
  )
);
array_column($a, 'last_name');
becomes
array_map(function($element){return $element['last_name'];}, $a);

This function does not preserve the original keys of the array (when not providing an index_key).
You can work around that like so:
<?php
// instead of
array_column($array, 'column');
// to preserve keys
array_combine(array_keys($array), array_column($array, 'column'));
?>

Some remarks not included in the official documentation.
1) array_column does not support 1D arrays, in which case an empty array is returned.
2) The $column_key is zero-based.
3) If $column_key extends the valid index range an empty array is returned.

<?php
# for PHP < 5.5 
# AND it works with arrayObject AND array of objects
if (!function_exists('array_column')) {
  function array_column($array, $columnKey, $indexKey = null)
  {
    $result = array();
    foreach ($array as $subArray) {
      if (is_null($indexKey) && array_key_exists($columnKey, $subArray)) {
        $result[] = is_object($subArray)?$subArray->$columnKey: $subArray[$columnKey];
      } elseif (array_key_exists($indexKey, $subArray)) {
        if (is_null($columnKey)) {
          $index = is_object($subArray)?$subArray->$indexKey: $subArray[$indexKey];
          $result[$index] = $subArray;
        } elseif (array_key_exists($columnKey, $subArray)) {
          $index = is_object($subArray)?$subArray->$indexKey: $subArray[$indexKey];
          $result[$index] = is_object($subArray)?$subArray->$columnKey: $subArray[$columnKey];
        }
      }
    }
    return $result;
  }
}
?>

array_column implementation that works on multidimensional arrays (not just 2-dimensional):
<?php
function array_column_recursive(array $haystack, $needle) {
  $found = [];
  array_walk_recursive($haystack, function($value, $key) use (&$found, $needle) {
    if ($key == $needle)
      $found[] = $value;
  });
  return $found;
}
Taken from https://github.com/NinoSkopac/array_column_recursive

My version is closer to the original than http://github.com/ramsey/array_column
<?php
/**
 * Provides functionality for array_column() to projects using PHP earlier than
 * version 5.5.
 * @copyright (c) 2015 WinterSilence (http://github.com/WinterSilence)
 * @license MIT
 */
if (!function_exists('array_column')) {
  /**
   * Returns an array of values representing a single column from the input
   * array.
   * @param array $array A multi-dimensional array from which to pull a
   *   column of values.
   * @param mixed $columnKey The column of values to return. This value may
   *   be the integer key of the column you wish to retrieve, or it may be
   *   the string key name for an associative array. It may also be NULL to
   *   return complete arrays (useful together with index_key to reindex
   *   the array).
   * @param mixed $indexKey The column to use as the index/keys for the
   *   returned array. This value may be the integer key of the column, or
   *   it may be the string key name.
   * @return array
   */
  function array_column(array $array, $columnKey, $indexKey = null)
  {
    $result = array();
    foreach ($array as $subArray) {
      if (!is_array($subArray)) {
        continue;
      } elseif (is_null($indexKey) && array_key_exists($columnKey, $subArray)) {
        $result[] = $subArray[$columnKey];
      } elseif (array_key_exists($indexKey, $subArray)) {
        if (is_null($columnKey)) {
          $result[$subArray[$indexKey]] = $subArray;
        } elseif (array_key_exists($columnKey, $subArray)) {
          $result[$subArray[$indexKey]] = $subArray[$columnKey];
        }
      }
    }
    return $result;
  }
}
?>

array_column() will return duplicate values. 
Instead of having to use array_unique(), use the $index_key as a hack.
**Caution: This may get messy when setting the $column_key and/or $index_key as integers.**
<?php
$records = [
    [ 'id' => 2135, 'first_name' => 'John' ],
  [ 'id' => 3245, 'first_name' => 'Sally' ],
  [ 'id' => 5342, 'first_name' => 'Jane' ],
  [ 'id' => 5623, 'first_name' => 'Peter' ],
    [ 'id' => 6982, 'first_name' => 'Sally' ]
];
print_r(array_unique(array_column($records, 'first_name')));
// Force uniqueness by making the key the value.
print_r(array_column($records, 'first_name', 'first_name'));
print_r(array_column($records, 'id', 'first_name'));
// Returns
/*
Array
(
  [0] => John
  [1] => Sally
  [2] => Jane
  [3] => Peter
)
Array
(
  [John] => John
  [Sally] => Sally
  [Jane] => Jane
  [Peter] => Peter
)
Array
(
  [John] => 2135
  [Sally] => 6982
  [Jane] => 5342
  [Peter] => 5623
)
*/
?>

If you need to extract more than one column from an array, you can use array_intersect_key on each element, like so:
function array_column_multi(array $input, array $column_keys) {
  $result = array();
  $column_keys = array_flip($column_keys);
  foreach($input as $key => $el) {
    $result[$key] = array_intersect_key($el, $column_keys);
  }
  return $result;
}

I added a little more functionality to the more popular answers here to support the $index_key parameter for PHP < 5.5
<?php
// for php < 5.5
if (!function_exists('array_column')) {
  function array_column($input, $column_key, $index_key = null) {
    $arr = array_map(function($d) use ($column_key, $index_key) {
      if (!isset($d[$column_key])) {
        return null;
      }
      if ($index_key !== null) {
        return array($d[$index_key] => $d[$column_key]);
      }
      return $d[$column_key];
    }, $input);
    if ($index_key !== null) {
      $tmp = array();
      foreach ($arr as $ar) {
        $tmp[key($ar)] = current($ar);
      }
      $arr = $tmp;
    }
    return $arr;
  }
}
?>

Here's a neat little snippet for filtering a set of records based on a the value of a column:
<?php
function dictionaryFilterList(array $source, array $data, string $column) : array
{
  $new   = array_column($data, $column);
  $keep   = array_diff($new, $source);
  return array_intersect_key($data, $keep);
 }
// Usage:
$users = [
  ['first_name' => 'Jed', 'last_name' => 'Lopez'],
  ['first_name' => 'Carlos', 'last_name' => 'Granados'],
  ['first_name' => 'Dirty', 'last_name' => 'Diana'],
  ['first_name' => 'John', 'last_name' => 'Williams'],
  ['first_name' => 'Betty', 'last_name' => 'Boop'],
  ['first_name' => 'Dan', 'last_name' => 'Daniels'],
  ['first_name' => 'Britt', 'last_name' => 'Anderson'],
  ['first_name' => 'Will', 'last_name' => 'Smith'],
  ['first_name' => 'Magic', 'last_name' => 'Johnson'],
];
var_dump(dictionaryFilterList(['Dirty', 'Dan'], $users, 'first_name'));
// Outputs:
[
  ['first_name' => 'Jed', 'last_name' => 'Lopez'],
  ['first_name' => 'Carlos', 'last_name' => 'Granados'],
  ['first_name' => 'John', 'last_name' => 'Williams'],
  ['first_name' => 'Betty', 'last_name' => 'Boop'],
  ['first_name' => 'Britt', 'last_name' => 'Anderson'],
  ['first_name' => 'Will', 'last_name' => 'Smith'],
  ['first_name' => 'Magic', 'last_name' => 'Johnson']
]
?>

//php < 5.5
if(function_exists('array_column'))
{
  function array_column($arr_data, $col)
  {
    $result = array_map(function($arr){return $arr[$col]}, $arr_data);
    return $result;
  }
}

if (!function_exists('array_column'))
{
  function array_column($input, $column_key=null, $index_key=null)
  {
    $result = array();
    $i = 0;
    foreach ($input as $v)
    {
      $k = $index_key === null || !isset($v[$index_key]) ? $i++ : $v[$index_key];
      $result[$k] = $column_key === null ? $v : (isset($v[$column_key]) ? $v[$column_key] : null);
    }
    return $result;
  }
}

Note that this function will return the last entry when possible keys are duplicated.
<?php
$array = array(
  array(
    '1-1',
    'one',
    'one',
  ),
  array(
    '1-2',
    'two',
    'one',
  ),
);
var_dump(array_column($array, $value = 0, $index = 1));
var_dump(array_column($array, $value = 0, $index = 2));
// returns:
/*
array (size=2)
 'one' => string '1-1' (length=3)
 'two' => string '1-2' (length=3)
array (size=1)
 'one' => string '1-2' (length=3)
*/
?>

Please note this function accepts 2D-arrays ONLY, and silently returns empty array when non-array argument is provided.
Code:
class testObject {
  public $a = 123;
}
$testArray = [new testObject(), new testObject(), new testObject()];
$result = array_column($testArray, 'a')); //array(0) { }

a simple solution:
function arrayColumn(array $array, $column_key, $index_key=null){
    if(function_exists('array_column ')){
      return array_column($array, $column_key, $index_key);
    }
    $result = [];
    foreach($array as $arr){
      if(!is_array($arr)) continue;
      if(is_null($column_key)){
        $value = $arr;
      }else{
        $value = $arr[$column_key];
      }
      if(!is_null($index_key)){
        $key = $arr[$index_key];
        $result[$key] = $value;
      }else{
        $result[] = $value;
      }
    }
    return $result;
  }

If array_column is not available you can use the following function, which also has the $index_key parameter:
if (!function_exists('array_column')) {
  function array_column($array, $column_key, $index_key = null) 
  {
    return array_reduce($array, function ($result, $item) use ($column_key, $index_key) 
    {
      if (null === $index_key) {
        $result[] = $item[$column_key];
      } else {
        $result[$item[$index_key]] = $item[$column_key];
      }
      return $result;
    }, []);
  }
}

Another option for older PHP versions (pre 5.5.0) is to use array_walk():
<?php
$array = array(
 array('some' => 'var', 'foo' => 'bar'),
 array('some' => 'var', 'foo' => 'bar'),
 array('some' => 'var', 'foo' => 'bar')
);
array_walk($array, function(&$value, $key, $return) {
 $value = $value[$return];
}, 'foo');
print_r($array);
// Array
// (
//   [0] => bar
//   [1] => bar
//   [2] => bar
// )
?>

This didn't work for me recursively and needed to come up with a solution. 
Here's my solution to the function:
if ( ! function_exists( 'array_column_recursive' ) ) {
  /**
   * Returns the values recursively from columns of the input array, identified by
   * the $columnKey.
   *
   * Optionally, you may provide an $indexKey to index the values in the returned
   * array by the values from the $indexKey column in the input array.
   *
   * @param array $input   A multi-dimensional array (record set) from which to pull
   *             a column of values.
   * @param mixed $columnKey The column of values to return. This value may be the
   *             integer key of the column you wish to retrieve, or it
   *             may be the string key name for an associative array.
   * @param mixed $indexKey (Optional.) The column to use as the index/keys for
   *             the returned array. This value may be the integer key
   *             of the column, or it may be the string key name.
   *
   * @return array
   */
  function array_column_recursive( $input = NULL, $columnKey = NULL, $indexKey = NULL ) {
    // Using func_get_args() in order to check for proper number of
    // parameters and trigger errors exactly as the built-in array_column()
    // does in PHP 5.5.
    $argc  = func_num_args();
    $params = func_get_args();
    if ( $argc < 2 ) {
      trigger_error( "array_column_recursive() expects at least 2 parameters, {$argc} given", E_USER_WARNING );
      return NULL;
    }
    if ( ! is_array( $params[ 0 ] ) ) {
      // Because we call back to this function, check if call was made by self to
      // prevent debug/error output for recursiveness :)
      $callers = debug_backtrace();
      if ( $callers[ 1 ][ 'function' ] != 'array_column_recursive' ){
        trigger_error( 'array_column_recursive() expects parameter 1 to be array, ' . gettype( $params[ 0 ] ) . ' given', E_USER_WARNING );
      }
      return NULL;
    }
    if ( ! is_int( $params[ 1 ] )
       && ! is_float( $params[ 1 ] )
       && ! is_string( $params[ 1 ] )
       && $params[ 1 ] !== NULL
       && ! ( is_object( $params[ 1 ] ) && method_exists( $params[ 1 ], '__toString' ) )
    ) {
      trigger_error( 'array_column_recursive(): The column key should be either a string or an integer', E_USER_WARNING );
      return FALSE;
    }
    if ( isset( $params[ 2 ] )
       && ! is_int( $params[ 2 ] )
       && ! is_float( $params[ 2 ] )
       && ! is_string( $params[ 2 ] )
       && ! ( is_object( $params[ 2 ] ) && method_exists( $params[ 2 ], '__toString' ) )
    ) {
      trigger_error( 'array_column_recursive(): The index key should be either a string or an integer', E_USER_WARNING );
      return FALSE;
    }
    $paramsInput   = $params[ 0 ];
    $paramsColumnKey = ( $params[ 1 ] !== NULL ) ? (string) $params[ 1 ] : NULL;
    $paramsIndexKey = NULL;
    if ( isset( $params[ 2 ] ) ) {
      if ( is_float( $params[ 2 ] ) || is_int( $params[ 2 ] ) ) {
        $paramsIndexKey = (int) $params[ 2 ];
      } else {
        $paramsIndexKey = (string) $params[ 2 ];
      }
    }
    $resultArray = array();
    foreach ( $paramsInput as $row ) {
      $key  = $value = NULL;
      $keySet = $valueSet = FALSE;
      if ( $paramsIndexKey !== NULL && array_key_exists( $paramsIndexKey, $row ) ) {
        $keySet = TRUE;
        $key  = (string) $row[ $paramsIndexKey ];
      }
      if ( $paramsColumnKey === NULL ) {
        $valueSet = TRUE;
        $value  = $row;
      } elseif ( is_array( $row ) && array_key_exists( $paramsColumnKey, $row ) ) {
        $valueSet = TRUE;
        $value  = $row[ $paramsColumnKey ];
      }
      $possibleValue = array_column_recursive( $row, $paramsColumnKey, $paramsIndexKey );
      if ( $possibleValue ) {
        $resultArray = array_merge( $possibleValue, $resultArray );
      }
      if ( $valueSet ) {
        if ( $keySet ) {
          $resultArray[ $key ] = $value;
        } else {
          $resultArray[ ] = $value;
        }
      }
    }
    return $resultArray;
  }
}

Retrieve multiple columns from an array:
$columns_wanted = array('foo','bar');
$array = array('foo'=>1,'bar'=>2,'foobar'=>3);
$filtered_array = array_intersect_key(array_fill_keys($columns_wanted,''));
//filtered_array 
// array('foo'=>1,'bar'=>2);

Value for existing key in the resulting array is rewritten with new value if it exists in another source sub-array.

Presented function is good when You want to flatten nested array base on only one column, but if You want to flatten whole array You can use this method:
/**
   * Method that transforms nested array into the flat one in below showed way:
   * [
   *   [
   *     [0]=>'today',
   *   ],
   *   [
   *     [0]=>'is',
   *     [1]=>'very',
   *     [2]=>  [
   *           [0]=>'warm'
   *         ],
   *   ],
   * ]
   *
   * Into:
   *
   * ['today','is','very','warm']
   *
   * @param $input
   * @return array
   */
  private function transformNestedArrayToFlatArray($input)
  {
    $output_array = [];
    if (is_array($input)) {
      foreach ($input as $value) {
        if (is_array($value)) {
          $output_array = array_merge($output_array, $this->transformNestedArrayToFlatArray($value));
        } else {
          array_push($output_array, $value);
        }
      }
    } else {
      array_push($output_array, $input);
    }
    return $output_array;
  }

<?php
if (!function_exists('array_column')) {
  function array_column($input, $column_key, $index_key = NULL) {
    if (!is_array($input)) {
      trigger_error(__FUNCTION__ . '() expects parameter 1 to be array, ' . gettype($input) . ' given', E_USER_WARNING);
      return FALSE;
    }
    
    $ret = array();
    foreach ($input as $k => $v) {    
      $value = NULL;
      if ($column_key === NULL) {
        $value = $v;
      }
      else {
        $value = $v[$column_key];
      }
      
      if ($index_key === NULL || !isset($v[$index_key])) {
        $ret[] = $value;
      }
      else {
        $ret[$v[$index_key]] = $value;
      }  
    }
    
    return $ret;
  }
}
?>