Check if username exists in database with AJAX

Question

So basically, I want to add a feature to my registration form which I will check if that username already exists in the database.

I have a few questions about AJAX -

Answer 1

1. if (mysql_num_rows(mysql_query("SELECT * FROM users WHERE username='".$_POST['username']."'")) < 1) { 
       true; 
   } else { 
       false; 
   }
   

2. you can pass it by get or post methods. in validation.php you just have to write $somevar=$_POST['somevar'];

3. as I just said you cant pass variables by post or get methods - to do this task, you must us GET.

i hope i was helpful ;)