FaceBook注册插件异步验证表单（以检查用户名是否可用）

Question

Ok, So I have Google the hell out of this problem and the FB Advanced Registration documentation also did not help. I want to have a Facebook Registration where a user can choose(& Check availability of) his username like this:
(Screenshot of what I plan to do but have failed to do, since I cant post picturesin this question directly) A link to Screenshot of what I wanted !

I plan to check the availability of the username from my DB in mysql using PHP, but I am stuck with this weird JSON callback thing which I have failed to understand. My Registration Plugin looks something like this

<fb:registration 
fields='[{"name":"name"},{"name":"username","description":"Username","type":"text"}]' 
onvalidate="validate_async"    
redirect-uri="http://mysite.com/loginFB.php"
fb_only="false"
width="530">

</fb:registration>    

 <script src="http://code.jquery.com/jquery-1.7.1.min.js"></script>
 <script> 
 function validate_async(form, cb) {
 // $.getJSON('https://graph.facebook.com/' + form.username + '?callback=?',//CODE obtained from FB documentation 
    $.getJSON('https://mysite.com/checkUsername.php?username=' + form.username + '?callback=?',
        function(response) {
          if (response.error== "false") {
        // Username isn't taken, let the form submit
        cb();
      }
      cb({username: 'That username is taken, Sorry!'});
  });
}
 </script> 

I wanted to know WHAT EXACTLY do I write the in the .
Right now I have come up with the following code for checkUsername.php which does NOT work:

 <?php 
 $conn = dbconnect(GLOBAL_Db);
$username = $_GET['username'];
$data = array();
$table = mysql_real_escape_string(GLOBAL_Db. "." . GLOBAL_Users);
 $sqlCommand = "SELECT * FROM ".$table." WHERE username='$username'";
 $query = mysql_query($sqlCommand) or die (mysql_error());
 $num_rows = mysql_num_rows($query);

if($num_rows>0){
    $data['error'] = "true";
} else {
$data['error'] = "false";
}
echo json_encode($data);
?>   

This code does not give me that that "The username is taken, Sorry" Message , WHY???
I would really appreciate it if anyone could actually help me out with this getJSON function in the script , AND Also help me out with the checkUsername.php since I have a very crude knowledge of JSON, (JSONP) etc. ! Would be glad to put in more effort to explaain my problem coz this is bugging me for like a Week now! Happy to accept some valuable help from you guys!

Answer 1

$username = $_GET('username');

$_GET is not a function … maybe you should familiarize yourself with some PHP basics first?

Besides that, your script logic looks a bit weird …

if($num_rows>0){
    $data['error'] = "true";
} else {

    $data['username'] = $row['username'];
}

If now rows are found, then you want to access the username field in a row that's not there ?

(And you're not even quoting the username you get as a GET parameter, OMG …)

---

Set error to true or false depending on if there are rows or none.

And then, in your JS function, give the „username is taken”-message if error=true.