Php on zend, how to escape a variable for a query?

im doing some queries in Zend Framework and i need to make sure no SQL injection is possible in the next kind of formats. I can use mysql_escape(deprecated) and wont do all the work. If i try to use real_mysql_escape it wont be able to grab the conection with the database and i cant find how zend_filter would solve the problem.

The query im doing (simplied) have the next sintaxes:

    $db = Zend_Registry::get('db'); 
    $select = "SELECT COUNT(*) AS num
                FROM message m
                WHERE m.message LIKE '".$username." %'";
    $row = $db->fetchRow($select);

What is the best way to prevent SQL INJECTION with this framework?