Given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.

smallest-positive-int-solution.php

/**
 * Finds the smallest positive integer which does not exist in an array.
 *
 * @param array $a
 *   The array to check.
 *
 * @return int
 *   The smallest positive integer not found in $a.
 */
function solution2(array $a) {
  // We only want the smallest positive number so we start at 1.
  $i = 1;

  // Iterate through positive integers until we reach the 1st number not in $a.
  while (in_array($i, $a)) {
    $i++;
  }

  return $i;
}

// This example will output: 5.
echo solution([1, 3, 6, 4, 1, 2]);

// This example will output: 4.
echo solution([1, 2, 3]);

// This example will output: 1.
echo solution([-1, -3]);

Your solution is true but has poor time complexity: O(N**2) which is not good. Thanks anyways

This one is O(N) or O(N * log(N))
function solution($A) {
sort($A);
$result = 0;
foreach($A as $ix) if ($ix > $result) if($ix == ($result + 1)) $result += 1;
return ($result +1);
}