Wow, this topic still has no proper answer :-)
If anybody will be interested, in PHP you just must assign everything which comes from a miniBB-form, in a $_POST array, they include bb_func_pthread.php or bb_func_ptopic.php, depends on the new reply or topic you would like to be posted. For example:
define ('INCLUDED776',1);
include($pathToOptions.'setup_options.php');
$genEmailDisable=1;
include($pathToFiles.'setup_mysql.php');
include_once($pathToFiles.'bb_functions.php');
include_once($pathToFiles.'bb_func_txt.php');
$_POST=array(
'topicTitle' => $title,
'postText' => $postText,
//'user_usr' => $val,
//'user_pwd' => $modzPwd,
//'mode' => 'login',
'queryStr' => '',
'forum' => $ft,
'topic' => $rndTopic,
'action' => 'pthread',
'anchor' => '0'
);
include($pathToFiles.'bb_func_pthread.php');