MySQL query update failed

Question

Please see the codes.

When I try:

function new_rate($item_selection,$service_selection,$rate)
    {
        try
        {                           
            $stmt = $this->conn->prepare("INSERT INTO rate(Item_Id,Service_Id,Rate) 
                                                         VALUES(:_item, :_service, :_rate)");
            $stmt->bindparam(":_item",$item_selection);
            $stmt->bindparam(":_service",$service_selection);
            $stmt->bindparam(":_rate",$rate);
            $stmt->execute();   
            return $stmt;
        }
        catch(PDOException $ex)
        {
            echo $ex->getMessage();
        }
    }


if(new_rate(2,8,550)){
                $msg = "Rate added successfully!";
                $success_msg = "";
            }
            else
        {
            $msg = "sorry , Query could not execute...";
        }

It works!

But when I try:

function update_query($table,$id,$update,$value)
    {
        try{
            $stmt = $this->conn->prepare("UPDATE ".$table." SET ".$update."=:_value WHERE Id=:_id");
            $stmt->bindparam(":_value",$value);
            $stmt->bindparam(":_id",$id);
            return $stmt->execute();
        }
        catch(PDOException $ex)
        {
            echo $ex->getMessage();
            return 0;
        }
    }

if(update_query("rate",5,"Rate",25)){
                $msg = "Rate updated successfully!";
                $success_msg = "";
            }
            else
        {
            $msg = "sorry , Query could not execute...";
        }

It displays: "Rate updated successfully!", but doesn't update the database. Where I am going wrong?

Please see the codes above?

Where I am going wrong?

Please see the codes above?