what is wrong with the function call

i have 2 php file (function.php, dp.php) in dp.php i call update function as follow

include('function.php');
$db = new Database();
$db->connect();
$db->update('classes', array('level'=> '(CASE WHEN level = 1 THEN 2
                WHEN level = 2 THEN 3
                WHEN level = 3 THEN 4                    
                END)','modifiedTime'=>date("Y-m-d H:i:s")), 'level IN (1,2,3)');
$res = $db->getResult();
print_r($res);

and the update function :

     public function update($table,$params=array(),$where){
    if($this->tableExists($table)){
        $args=array();
        foreach($params as $field=>$value){
            $args[]=$field.'="'.$value.'"';
        }
        $sql='UPDATE '.$table.' SET '.implode(',',$args).' WHERE '.$where;          
        $this->myQuery = $sql; // Pass back the SQL
        if($query = $this->myconn->query($sql)){
            array_push($this->result,$this->myconn->affected_rows);
            return true; 
        }else{
            array_push($this->result,$this->myconn->error);
            return false;
        }
    }else{
        return false;
    }
}

the result i always get is 0 ,where it should be 2, 3, or 4.

the actual statement the is working when it is executed phpmyadmin is:

    update classes set level = (CASE WHEN level = 1 THEN 2
                WHEN level = 2 THEN 3
                WHEN level = 3 THEN 4                    
                END), modifiedTime = date("Y-m-d H:i:s")) WHERE level IN (1,2,3));

thanks in advance