为何PHP不能接收。

 $.ajax({
       type: "POST",
        url: "index.php",
        data: "location=img_png_src",
        success: function(msg){
         alert( "Data Saved: " + msg );
           }
      });

if($_SERVER['REQUEST_METHOD'] == "POST") {
  print_r($_POST);
  exit();
}
?>