php合並兩個二維數組

$a = array(0 => Array(id => 66,class_name => math),1 => Array(id => 67,class_name => linux ));
$b = array(0 => Array(class_count=> 8),1 => Array(class_count => 2));
$arr = array();
foreach($a as $k=>$r){
   $arr[] = array_merge($r,$b[$k]);
     }