- array_search: documentation ( source)
- array_keys: documentation ( source)
- max: documentation ( source)
<?php
$array = [
3 => 1,
7 => 2,
5 => 3,
2 => 4,
0 => 5
];
echo max(array_keys($array)); // 4
echo sizeof($array)-1; // 4
echo array_search(max($array), $array); // 4