Cannot complete query to db

Hello,
I want to develop my own snippet in ModX, but cannot complete any query to db through xPDO.
There's a working site with wotking gallery already.
$modx->getManager(); works, but when i try
$m=$modx->getManager();
$create= $m->createObjectContainer('example_table'); it cannot create anything
The same is about $modx->getCollection('already_used_table');and anything else.
When i try direct php-query through mysq_connect and mysql_query i get what i want.

Also when i copy some db-query code from gallery it doesn't work.

Just don't get where the problem is.
Please anybody help.