Null when update Date field - MySQL PHP -

i'm trying update date field in db. query this:

$q = "update news set data = str_to_date('2011-03-05','%y-%m-%d'), title = '".$title."', content='".$content."',...."; 

works great, but:

$q = "update news set data = str_to_date('".$data."','%y-%m-%d'), title = '".$title."', content='"..."; 

it not working :(

i got date:

$data = $_post["data"]; 

and has value "2013-04-13". trimmed date , show in popup window , value correct; plz :)

update strange me, if i'm using:

$q = "insert news set data = cast('".$data."' date), title = '".$title."', content='".$content."'..."; 

it works fine. in insert not in update

script table:

create table if not exists `news` (`id` int(11) not null auto_increment, `data` date not null, `title` text not null, `content` text not null, `img` text not null, `n_img` text not null, primary key (`id`), key `id` (`id`)) engine=myisam  default charset=utf8 auto_increment=36 ; 

why using str_to_date? looks date in correct format. try removing , inserting is?

might want escape first though.

$data = mysql_real_escape_string($_post["data"]);