Using PHP key() to Retrieve Keys from Associative Arrays

The key() function returns the key name of the current element in an array without moving the internal pointer. If the pointer is beyond the end of the array or the array is empty, it returns NULL.

Parameter: array – the associative array to operate on.

Return value: The key of the current array element, or NULL if no element is selected.

Example:

<?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);
}
?>

The script iterates through $array, checks each value, and prints the corresponding key when the value equals "apple".

Output:

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