I would like to accomplish the following:

If a username or password field is null, notify the user. If user name already exists, do not insert into the database and notify user to create a different name. if the username is unique and password is not null, return the username to the user. As of now it always returns "Please enter a different user name." I believe the issue has to do with the database query but I am not sure. If anyone can have a look and see if I am making an error, I greatly appreciate it, thanks.

if ($userName or $userPassword = null)
{

  echo "Please enter a user name and password or return to the homepage.";

}

elseif (mysql_num_rows(mysql_query("SELECT count(userName) FROM logininfo WHERE userName = '$userName'")) ==1)

{

  echo "Please enter a different user name.";

}

elseif ($userName and $userPassword != null)

{

  echo "Your login name is: $userName";

}