Copying x random values from one array to another PHP

I'm trying to copy 5 random values from one array to another with the following code. The problem is 3 or 4 values get copied and 1 or 2 are always copied as null. I'm not sure what the problem in my code is.

if (count($potential_matches_in_area) >= 5) {
  for ($x = 0; $x < 5; $x++) {

  $index = mt_rand(0, count($potential_matches_in_area) - 1);
  $new_matches[$x] = $potential_matches_in_area[$index];
  unset($potential_matches_in_area[$index]);

  }