All you need is to generate many adresses from a single seed. Store those addresses in some data structure or csv file.

Then each time user requests deposit address - pick one of the pregenerated addresses from stored pool and assign it to respectable user (i.e. remove it from the address pool and add some field in your users database,  put address there).

That's all.


Don't generate address every time user needs a new one. It will be unsafe since to actually generate address you need access to your private key. It is much more safe to do it just once and then use your addresses as a text file. There is no limit on how much addresses you can generate so you can pregenerate as much as you want for all your existing or new users. Once it is done - you can handle deposits/withdrawals separately.