check if MySQL return result in PHP using mysql_num_rows

In PHP, I have following query to check if userid exist
 $result = mysql_query("SELECT USERID,  FIRSTNAME, LASTNAME FROM USERS WHERE USERID=".$userid);  
if userid exists,
if (mysql_num_rows($result)>0) {echo "user already in database";}
Here PHP function mysql_num_rows is used  to get how many rows returned from MySQL query.