post category in wp_insert_post

I did not actually test your code thoroughly, but on the first glimpse:
your form input is named post_category while you’re grabbing $_POST['new_cat']. Adjusting that (e.g., set both to post_category) should already do.

// EDIT
And why are you grabbing $_POST['category'] in line 5?

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)