Using isset to achieve this, is many times faster:

<?php

 $m = range(1,1000000);
  $s = [2,4,6,8,10];

// Use array_intersect to return all $m values that are also in $s
  $tstart = microtime(true);
  print_r (array_intersect($m,$s));
  $tend = microtime(true);
  $time = $tend - $tstart;
  echo "Took $time";
  
 // Use array_flip and isset to return all $m values that are also in $s
  $tstart = microtime(true);
  $f = array_flip($s);
/* $f now looks like this:
(
    [2] => 0
    [4] => 1
    [6] => 2
    [8] => 3
    [10] => 4
)
*/
// $u will hold the intersected values
  $u = [];
  foreach ($m as $v) {
    if (isset($f[$v])) $u[] = $v;
  }
  print_r ($u);
   $tend = microtime(true);
  $time = $tend - $tstart;
  echo "Took $time";
?>

Results:

Array
(
    [1] => 2
    [3] => 4
    [5] => 6
    [7] => 8
    [9] => 10
)
Took 4.7170009613037
(
    [0] => 2
    [1] => 4
    [2] => 6
    [3] => 8
    [4] => 10
)
Took 0.056024074554443

array_intersect: 4.717
array_flip+isset: 0.056