Find missing value in array

$a = array(4,6,8,9,10,12,14,16,17,18,20,21,22,23,24,25,26,27,29,31,32,33,35,36,38,39,40,41,43,44,45,46,48,50,52,53,56);
$missing= array();
for($i=$a[0];$i