key()

(PHP 4, PHP 5, PHP 7)

从关联数组中取得键名

说明

key(array $array): mixed

key()返回数组中当前单元的键名。

参数

$array

该数组。

返回值

key()函数返回数组中内部指针指向的当前单元的键名。但它不会移动指针。如果内部指针超过了元素列表尾部，或者数组是空的，key()会返回NULL。

更新日志

范例

Example #1key()例子

<?php
$array = array(
    'fruit1' => 'apple',
    'fruit2' => 'orange',
    'fruit3' => 'grape',
    'fruit4' => 'apple',
    'fruit5' => 'apple');
// this cycle echoes all associative array
// key where value equals "apple"
while ($fruit_name = current($array)) {
    if ($fruit_name == 'apple') {
        echo key($array).'<br />';
    }
    next($array);
}
?>

以上例程会输出：

fruit1<br />
fruit4<br />
fruit5<br />

参见

• current() 返回数组中的当前单元
• next() 将数组中的内部指针向前移动一位
• foreach

Note that using key($array) in a foreach loop may have unexpected results. 
When requiring the key inside a foreach loop, you should use:
foreach($array as $key => $value)
I was incorrectly using:
<?php
foreach($array as $value)
{
 $mykey = key($array);
}
?>
and experiencing errors (the pointer of the array is already moved to the next item, so instead of getting the key for $value, you will get the key to the next value in the array)
CORRECT:
<?php
foreach($array as $key => $value)
{
 $mykey = $key;
}
A noob error, but felt it might help someone else out there.

Suppose if the array values are in numbers and numbers contains `0` then the loop will be terminated. To overcome this you can user like this
<?php
$array = array(
  '0' => '5',
  '1' => '2',
  '2' => '0',
  '3' => '3',
  '4' => '1');
// wrong approach
while ($fruit_name = current($array)) {
    echo key($array).'<br />';
    next($array);
}
// the way will be break loop when arra('2'=>0) because its value is '0', while(0) will terminate the loop
// correct approach
while ( ($fruit_name = current($array)) !== FALSE ) {
    echo key($array).'<br />';
    next($array);
}
//this will work properly
?>

Needed to get the index of the max/highest value in an assoc array.
max() only returned the value, no index, so I did this instead.
<?php
reset($x);  // optional.
arsort($x);
$key_of_max = key($x);  // returns the index.
?>

(Editor note: Or just use the array_keys function)
Make as simple as possible but not simpler like this one :)
$k = array();
for($i = 0; $i < count($arr); $i++){
  $k[$i] = key($arr);
  next($arr);
}

In addition to FatBat's response, if you'd like to find out the highest key in an array (assoc or not) but don't want to arsort() it, take a look at this:
<?php
$arr = [ '3' => 14, '1' => 15, '4' => 92, '15' => 65 ];
$key_of_max = array_search( max($arr) , $arr);
?>