Your php.ini file will depend on what type of server you are on. Is this your server or a hosted server?

If you just want to disable the check then you can edit addpost.php. About line 126 you'll see this code block:

if(!$form_check_ok) {
$html->not_right($ubbt_lang["INVALID_FORM_KEY"]);
} // end if


Just remove that block.