Order ID

Hello,

Does anybody know how to change (somewhere in settings or some php files) how would be Order ID generated? Now, it consist of numbers and letters (looks like some random string), and my customer need just numbers (because, he will use Order ID in payment parameters, and banks in our Country don't accept letter in that parameter). So, it can be random string... but it have to consist just from numbers.

Thank you very much.

Kindest regards,

Sinisa Sinjori