Question

The var $username needs to check for a match. How can I do this?

Progress:

if (isset($_GET["username"]) && !empty($_GET["username"])) 
{
    $username = $_GET['username'];

    $usercheck = mysql_query("SELECT * FROM wp_users WHERE user_login=".$username."",$con);
    closeCursor($usercheck);

Answer 1

Do it like this:

1) Escape the variable to prevent SQL injection using mysql_real_escape_string.
2) Use quotes around the variable in where clause, because it is a string.
3) Check whether more than 0 rows were returned or not using mysql_num_rows.

 $username = mysql_real_escape_string($_GET['username']);
 $usercheck = mysql_query("SELECT * FROM wp_users WHERE user_login='".$username."'",$con);

 if(mysql_num_rows($usercheck)>0)
      echo 'USER FOUND';
 else
      echo 'NOT FOUND';