php - PDO bound parameters causing incorrect results with AES_ENCRYPT

Question

I have a problem using the function AES_ENCRYPT with PDO, when I use the SQL statement below with :pass and :key as bound parameters, I get no match from the database. However if I take them out and put them directly in the SELECT statement I get a COUNT of 1 result which is correct.

Am I embedding these incorrectly into the statement, or is there another way to make my select function work properly with the AES_ENCRYPT function? The function works correctly with straight parameters, for example

SELECT * FROM users WHERE name=:name AND email=:email

$query[] = "SELECT COUNT(1) FROM users WHERE active=1 AND username=:username AND password=AES_ENCRYPT(:pass,:key);";
$query[] = array( 
            'username'    => $username,
            'pass'        => $pass,
            'key'         => $key );
list($exists) = select( $query, true );

function select( array &$query, $one=FALSE ) {

    try {

      $stmt = $this->PDO->prepare( $this->named_queries( $query[0] ) );
      foreach( $query[1] as $k=>$v ) {
        $stmt->bindParam( ":$k", $v );
      }
      $stmt->execute();
      $query = array(); # If the query is from a loop this prevents only the first loop's SELECT from being used.

      if( $one ) {
        $return = $stmt->fetch();
      }else{
        $return = $stmt->fetchAll();
      }
      return $return;

    }catch( PDOException $e ) {

      if( $this->test )
        self::print_error( __METHOD__, $e->getMessage() );

      return false;
    }
  }