PHP 2個數組做對比並輸出數組中不存在的值

<?php
	
	
  $a = array("1","2","3","4","5","6","7","8");
  
  $b = array("1","7","8");
  
  $c = array_merge(array_diff($a,$b),array_diff($b,$a));
  
  print_r($c);

?>