PermMissingElem.php

<?php
/*
Find the missing number in a range (array)
*/
function permMissingElem(array $arr) : int
{
    $n = count($arr) + 1;
    $num = (($n + 1) * $n) / 2;
    
    foreach($arr as $val) {
        $num = $num - $val;
    }
    
    return $num;
}

$test = [2, 3, 1, 5]; // 4

var_dump(permMissingElem($test));