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Showing content from https://www.php.net/manual/en/function.array-find-key.php below:

PHP: array_find_key - Manual

array_find_key

(PHP 8 >= 8.4.0)

array_find_key — Returns the key of the first element satisfying a callback function

Description

array_find_key() returns the key of the first element of an array for which the given callback returns true. If no matching element is found the function returns null.

Parameters

array: The array that should be searched.
callback: The callback function to call to check each element, which must be
If this function returns true, the key is returned from array_find_key() and the callback will not be called for further elements.

Return Values

The function returns the key of the first element for which the callback returns true. If no matching element is found the function returns null.

Examples

Example #1 array_find_key() example

<?php

    $array = [
'a' => 'dog',
'b' => 'cat',
'c' => 'cow',
'd' => 'duck',
'e' => 'goose',
'f' => 'elephant'
];// Find the first animal with a name longer than 4 characters.
var_dump(array_find_key($array, function (string $value) {

    return strlen($value) > 4;

    }));// Find the first animal whose name begins with f.
var_dump(array_find_key($array, function (string $value) {

    return str_starts_with($value, 'f');

    }));// Find the first animal where the array key is the first symbol of the animal.
var_dump(array_find_key($array, function (string $value, $key) {

    return $value[0] === $key;

    }));// Find the first animal where the array key matching a RegEx.
var_dump(array_find_key($array, function ($value, $key) {

    return preg_match('/^([a-f])$/', $key);

    }));
?>

The above example will output:

string(1) "e"
NULL
string(1) "c"
string(1) "a"