Ajax内部错误，jqXHR: undefined

我修改了我的代码，试图找到这个bug。我有一个Ajax post函数在我的代码的其他地方工作，它工作得很好。

由于某种原因，这个人不想合作。

我要减少输入的数量，这样代码看起来不会太长。这里是php

<?php
// --------Connect to DB --->
include 'connect.php';
$conn = connect ();
//====================================================================>
//grab data 
$this_id = $_POST('pid_num');
$this_start_date = $_POST('date_ammend');   
$sql_update_query = "UPDATE Galaxy_jobs SET date ='$this_start_date' WHERE PID = $this_id";
//====================================================================>
mysqli_query ($conn,$sql_update_query); 
//close
mysqli_close($conn);
?>

function ammend_job()
    {
        pid = '2'; // test figures
        start_date_ammended = '11-11-1111';  // test figures
        var Data = {
               pid_num : pid, 
               date_ammend : start_date_ammended,
        };
        $.ajax({
            url:"ammend_job.php",
            type: "POST",
            dataType: 'text',
            data: Data,
             success: function(data){ 
                if(data.status == 'success')
                    alert('Post has been uploaded to Database');
                }, 
            error: function(xhr,textStatus,err,jqXHR) {
            console.log("readyState: " + xhr.readyState);
            console.log("responseText: "+ xhr.responseText);
            console.log("status: " + xhr.status);
            console.log("text status: " + textStatus);
            console.log("error: " + err);
            console.log("Jquery error:" + jqXHR)
          // alert('There is an error, screenshot this error and send to Admin : TextStatus: ' +textStatus+"  -  Error:  "+errorThrown+" - XMLRequest: "+XMLHttpRequest+"-  Response Text"+xhr.responseText);
           }
        });
    }

错误码如下:

main.php:102 readyState: 4
main.php:103 responseText: 
main.php:104 status: 500
main.php:105 text status: error
main.php:106 error: Internal Server Error
main.php:107 Jquery error:undefined