For PHP <= 7.3.0 :

if (! function_exists("array_key_last")) {
    function array_key_last($array) {
        if (!is_array($array) || empty($array)) {
            return NULL;
        }
        
        return array_keys($array)[count($array)-1];
    }
}

For those who loves tested candies;

<?php
function array_key_last(array $array) {
    // 100000 iters: ~0.068 secs
    return key(array_slice($array, -1));
    // 100000 iters: ~0.070 secs
    return key(array_reverse($array));
    // 100000 iters: ~0.088 secs
    return array_keys($array)[count($array) - 1] ?? null;
}
?>

Try to beat this polyfill in terms of performance!

<?php
if( !function_exists('array_key_last') ) {
    function array_key_last(array $array) {
        if( !empty($array) ) return key(array_slice($array, -1, 1, true));
    }
}

// Bonus
if (!function_exists('array_key_first')) {
    function array_key_first(array $arr) {
        foreach($arr as $key => $unused) return $key;
    }
}
?>

https://www.php.net/manual/fr/function.array-key-last.php#123950

did return the first key of my array only:
"array_keys($array)[count($array) - 1] ?? null" works

I hope the example is fine for everyone else.

My array is:
[
            new NameExpression('d', 0),
            new NameExpression('f', 0),
            new NameExpression('g', 0),
]

// if : php ver < 7 
// array_key_last 

$arr = array('1'=>'bear1','2'=>array('22'=>'bear22'),'3'=>'bear3');
echo end(array_keys($arr));

// result : 3

<?php 
    // PHP >= 7
    function _array_key_last(array $array){
        return (!empty($array)) ? array_keys($array)[count($array)-1] : null;
    }
    var_dump(_array_key_last(['PHP', 'Javascript', 'Python'])); // 2
 ?>

For PHP < 7.3.0 :

Will work for any type of array

<?php
if ( ! function_exists( 'array_key_last' ) ) {
    /**
     * Polyfill for array_key_last() function added in PHP 7.3.
     *
     * Get the last key of the given array without affecting
     * the internal array pointer.
     *
     * @param array $array An array
     *
     * @return mixed The last key of array if the array is not empty; NULL otherwise.
     */
    function array_key_last( $array ) {
        $key = NULL;

        if ( is_array( $array ) ) {

            end( $array );
            $key = key( $array );
        }

        return $key;
    }
}
?>

Notice!

$a = array(
    1 => 1,
    0 => 0
);

echo (array_key_last($a)); // output: "0", not "1"

Now that we have this function, where we once used...
<?php
$idxLast = count($arr) - 1;
?>
we can now optimize code speed by using...
<?php
$idxLast = (array_key_last($arr) ?? 0);
?>
Use-cases include lots of looping and control-structures.
If you do not know ahead of time that the array is numerically indexed, and that it could possibly be associative...safer/faster to stick with count().

This polyfill works for PHP 5.6+. 
It is a slight modification of "wes at nospam dot example dot org"'s example:

<?php

    if( !function_exists('array_key_first') ) {

        function array_key_first(array $array) {

            if( $array === [] ) { return NULL; }

            foreach($array as $key => $_) { return $key; }
        }
    }

    if( !function_exists('array_key_last') ) {

        function array_key_last(array $array) {

            if( $array === [] ) { return null; }
             
            // the last 2 args to array_slice are crucial
            return array_key_first(array_slice($array, -1, null, true));
        }
    }

Here come my benchmark, and 

return key(array_slice($array, -1));

is the my best answer for a polyfill.

<?php

function array_key_last_1(array $array) {
    // assert(!empty($array));
    return key(array_slice($array, -1));
}

function array_key_last_2(array $array) {
    return key(array_slice($array, -1, 1, true));
}

function array_key_last_3(array $array) {
    return key(array_reverse($array));
}

function array_key_last_4(array $array) {
    return array_keys($array)[count($array) - 1] ?? null;
}

function array_key_last_5(array $array) {
    return array_keys($array)[count($array) - 1] ?? null;
}

function array_key_last_6(array $array) {
    $copy = array_keys($array);
    return end($copy);
}

$result = array();
$array = range(0, 1000000);

for($i=1; $i < 7;$i++) {
  $callback = 'array_key_last_' . $i;
  $before = microtime(true);
  call_user_func_array($callback, array($array));
  $after = microtime(true);
  $time = ($after-$before);
  $result[$callback] = $time;
}

asort($result);
foreach ($result as $key => $value) {
    echo '[' . $key . ']' . $value . " sec" . PHP_EOL;
}

Correct polyfill (one that doesn't make copies of arrays, and that doesn't make use of the IAP) is:

<?php

if(!function_exists('array_key_first')){
    function array_key_first(array $array){
        if($array === []){
            return NULL;
        }
        foreach($array as $key => $_) return $key;
    }
}

if(!function_exists('array_key_last')){
    function array_key_last(array $array){
        if($array === []){
            return NULL;
        }
        return array_key_first(array_slice($array, -1));
    }
}

?>