PHP中计算二维数组中某一元素之和

$str=array(  
    0=>array( "flag" => "1" , "look" => "哈哈" ),  
    1=>array( "flag" => "2" ,  "look" => "哈哈" )  
); 

$sum = 0;
foreach($str as $item){
  $sum += (int) $item["flag"];
}  
 
var_dump($str);
echo $sum; die();