Codility-lesson3-PermMissingElem.php

<?php

function solution($A) {
    // write your code in PHP5.5
    $count = count($A);
    if ($count < 1) return 1;

    $end = $count + 1;
    $ordered = range(1, $end);

    $flip_A = array_flip($A);
    $flip_ordered = array_flip($ordered);

    $diff = array_diff_key($flip_ordered, $flip_A);

    if(count($diff) == 0) return 1;

    $key_flip = current($diff);
    $value = $ordered[$key_flip];
    return $value;
}