Check if username exist, then i want to update - php

Question

I have a code that checks if the username is available if you want to change. But now I saw that if you update something else such as your password, you can assume to save as the user name already exists.
Listed below are the code I use, as you see, I have tried to think of something but did not go well at all.
PHP
$sql = "Select * FROM table WHERE Slug = '$slug' AND ID ='$id' LIMIT 1";
$query = mysql_query($sql);
if(mysql_num_rows($query)>0){
echo 'true';
}else{
$e_sql = "Select * FROM table WHERE Slug = '$slug'";
$e_query = mysql_query($e_sql);
if(mysql_num_rows($e_query)>0){
echo 'false';
}else{
echo 'false';
}
}
Jquery/Javascript
var response;
$.validator.addMethod(
"uniqueUserName",
function(value, element) {
$.ajax({
type: "POST",
url: "User.php",
data: {
'slug': value,
'id': <?php echo $id; ?>
},
dataType:"html",
success: function(msg)
{ console.log(msg);
response = ( msg == 'true' ) ? true : false;
}
});
return response;
},
"This Name is already used!"
);
$("#addSurvey").validate({
rules: {
name: {
required: true,
uniqueUserName: true
},
}
});

Answer 1

Basically problem is in your $.ajax request.
As you know $.ajax by default perform an asynchronous HTTP (Ajax) request.
Easiest solution for you is to make request synchronous.
To make request synchronous you should set option async: false - in this case code will be executed linearly and you will get return response; only when ajax request is completed.
So basically change part of ajax call to:
$.ajax({
type: "POST",
url: "User.php",
async: false,
data: {
'slug': value,
'id': <?php echo $id; ?>
},
dataType:"html",
success: function(msg){
response = msg === 'true';
}
});
May be useful:
jQuery.ajax() Documentation

Answer 2

set
response = (( msg == 'true' ) ? true : false);