This empty string won't insert in my database

[eluser]jtkendall[/eluser]
That's because it's not NULL. is_null on an empty object will return false. My suggestion would be to modify your code to check if it's an object as well:

Code:

$reply = (is_null($t->in_reply_to_user_id) || is_object($t->in_reply_to_user_id)) ? 'NULL' : $t->in_reply_to_user_id);

This way if it's not null and is an object you set it as null, however if it's a string or integer it will return that string or integer.