I am new in bitcoin. I am developing a PHP website to let users build their wallet on my website. I want to give every user a wallet. I do not want to give users private keys. I keep private keys, so all their bitcoins owns mine. If everyone wants to spend his/her bitcoins, he/she needs to send a withdrawal request and I will handle it. I want to be able to send user’s bitcoin at once with one transaction and one fee. So if I give every user a new address, bitcoins will be a changeed (a little amount in every address). So spending them needs lots of fee. What should I do? I am using blockcypher’s API. Is there any other API can help? What senario should I do to achieve all above targets? thanks.