Solution for Missing integer problem of codility in php 100% score

Question

Problem Find the smallest positive integer that does not occur in a given sequence.

So what is the best implementation in PHP for this problem of codility!

Solution below results 66%, causing performance issue.

function solution($A)
{
    sort($A);
    $end = count($A);
    $flag = false;
    for ($k = 0; $flag == false; $k++, $flag = false) {
        for ($i = 0; $i < $end; $i++) {
            if ($k + 1 == $A[$i]) {
                $flag = $A[$i];
                break;
            }
        }
        if($flag == false){
            return $k +1;
        }
    }
}

Answer 1

A simple solution using an associative array as a set:

function solution($A) {
    $set = array_flip($A);
    for ($n = 1; ; ++$n) {
        if (!isset($set[$n])) {
            return $n;
        }
    }
}