array_is_list($ary) returns true if $ary is a sequential array and false if is an associative array. <?php
$list = [1,2,3,4];
$dict = ['num' => 42, 'txt' => 'Hello world'];
if (array_is_list($list)) { print("\$list is a list\n"); }
if (array_is_list($dict)) { print("\$dict is a list\n"); }
?>