关于backbone+php的destroy问题

我通过重写backbone sync的destroy（）来对应到php(点击删除按钮会触发delete事件)，在php里实现对数据的删除，但是因为collection里定义了url，在重写destroy的url时，无法执行相应操作，求解答！

//Collection
App.Users=Backbone.Collection.extend({
  model : App.User,
  url : function(){
    return '/crud2/save.php';
  },
  initialize : function(){
    this.on('add',function(){
      console.log(this.length);
    });
    this.on('remove',function(){
      console.log(this.length);
    });
  }
});

  `delete : function(){
  //this.model.url='delete.php';
  //console.log(this.model.url);
  $('#Name').val("");
  $('#Phone').val("");
  this.model.destroy({
    url : 'delete.php',
    type : 'POST',
    success : function(){
      console.log('success');
    },
    error : function(){
      console.log('error');
    }
  }
  ); 
}`

//delete.php
<?php
    date_default_timezone_set('prc');
    error_reporting(E_ALL ^ E_DEPRECATED);
    header("Content-type : application/json; charset = utf-8");  
    $connection=mysql_connect("localhost","root","123456");
    if(!$connection){
        die('connect failed!');
    }else{
        mysql_select_db("backbone");
        mysql_query("set names utf-8");
        $data = json_decode(file_get_contents("php://input"));
        //echo $data;
        file_put_contents("data.txt", $data->username);
        $sql = "delete from user where username == '$data->username'";
        mysql_query($sql);
?>