PHP error mysql_num_rows returns expects boolean error. is my query wrong?
<p>mysql_num_rows() expects parameter 1 to be resource, boolean given in line 33</p>

<pre><code>if($password!=$password_again){
            echo'password did not match';
        }else{
            $query="SELECT username FROM users WHERE 'username'='$username'";
            $query_run=mysql_query($query);


            if(mysql_num_rows($query_run)==1){
                echo'the username'.$username.'already exists';  
            }else{
                echo'ok';
            }
        }
</code></pre>
