为什么jQuery AJAX失败了PHP？

This doesn't work

$('#signup').submit(function(event) {
event.preventDefault();
var url = $(this).attr("action");
var data = $(this).serialize();
var callback = function(response) {$("#result").html(response);};
$.post(url, data, callback);
})

Try the below code in your form page with the JavaScript posted in your question:

<form id="signup" action="test_action.php">
<table width="300" cellpadding="3" cellspacing="0" border="0">
<tr><td>Name</td><td><input type="text" name="name" value="MY name" /></td></tr>
<tr><td>Email</td><td><input type="text" name="email" value="user@domain.com" /></td></tr>
<tr><td>&#160;</td><td><input type="submit" value="Post" /></td></tr>
<tr><td>&#160;</td><td id="result">&#160;</td></tr>
</table>
</form>

Create a page 'test_action.php' with the below content for the form submit.

<?php
echo "success!";
?>

If you see the text 'success!' in the 'result' element, after the Post button click, the form submit works.

 echo "Please fill in all of the fields!";
         $errors = $emailrrors + 1;


     }
$u_check = mysqli_query($mysqli,"SELECT un FROM users WHERE un='$un'");

     $check = mysqli_num_rows($u_check);

    if ($check===1)
     {
     echo "Username    already taken ...";
     $ok = 0;
     $errors = 1;
     }
     $email_check = mysqli_query($mysqli,"SELECT email FROM users WHERE email='$email'");

     $emailcheck = mysqli_num_rows($email_check);

    if ($emailcheck===1)
     {
     echo "email    already taken ...";
     $ok = 0;
     $errors = 1;
     }
      if($errors==0)

      {
if(!$mail->send()) {
    echo 'Message could not be sent.';
    echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
    echo 'Message has been sent';
}
if($errors!=0)
    {
        echo 'something went wrong';
        }