codility-permutation-check.php

<?php

$a = [1, 2, 3, 4, 6, 7];

sort($a);

if ($a[0] != 1) {
    return 0;
}

if ($a[0] < 1) {
    return 0;
}

$supposed = $a[0] + (count($a) - 1);

$isMissing = $supposed != $a[count($a) - 1];

return $isMissing ? 0 : 1;