php sql无法正常工作

The query isn't updating, not sure what I've done wrong.

$q_result = mysql_query("UPDATE 
                           users 
                         SET
                           recovery_answer = '".$_POST['answer']."', 
                           recovery_question = '".$_POST['question']."',  
                           date_question_set = '" . mysql_real_escape_string($register_date) . "' 
                         WHERE
                           username='" . mysql_real_escape_string($_SESSION['username']) . "'");

Can you echo "UPDATE users SET recovery_answer = '".$_POST['answer']."', recovery_question = '".$_POST['question']."',
date_question_set = '" . mysql_real_escape_string($register_date) . "' WHERE username='" . mysql_real_escape_string($_SESSION['username']) . "'" and post?