调用ajax函数返回错误

我有以下代码ajax上的html按钮点击。

$(document).ready(function(){
  $('#button').click(function() {
     //alert($("#data").serialize());
     $.ajax({
           type:"post",
           url:"geofenceupdatedata.php",
           data:  $("#data").serialize(),
           success: function(data){
                alert("Success");               
           },
            error:function(XMLHttpRequest, textStatus, errorThrown) { 
                alert("Status: " + textStatus.Message+errorThrown+XMLHttpRequest); 
                //alert("Error: " + errorThrown); 
            }
     });
  });
});

<?php
// receive data from app's http request
include_once'./connectionusers.php';
echo var_dump($_POST);
$name=$_POST["value1"];
$lat=$_POST["value2"];
$latitude=(double)$lat;
$long=$_POST["value3"];
$longitude=(double)$long;
$rad=$_POST["value4"];
$radius=(double)$rad;
$message=$_POST["value5"];
$username="admin";
    $date=date("%c");
// write data from my webapp to a text file
file_put_contents('update.txt',$name." 'n".$message);  
    $result=pg_query("UPDATE geofence  SET latitude='$latitude', longitude='$longitude', datetime='$date',         message='$message',        username='$username', radius='$radius' WHERE name='$name';");
 $js=json_encode(array("result"=>"success"));
 echo $js;
 return $js;
?>

ajax代码调用php代码并更新数据库，但没有调用ajax的成功部分。误差函数为。它为什么要这么做?