title: Check if username already exists in database MySQL PHP
body: <p>I have a registration.php page I made and I need to check if a username is already in my database so that I won't have 2 members with the same name...</p>
  
  <p>Here is how I am inserting the user info into the database (username, password, date of registration, etc.):</p>
  
  <pre><code>mysql_query("INSERT INTO UserDb (ID, TimeStamp, UserName, Password) VALUES ('$ipaddress', NOW(), '$user_username', '$user_password')");
  </code></pre>
  
  <p>How can I check that database "UserDb" under the "UserName" field to make sure the user's variable "$user_username" doesn't already exist?</p>