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;
    }
}
?>