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

Question

what is the best implementation in PHP for missing integer 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

Here is the Best solution for the codility problem implemented in PHP, scoring 100%

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