thank you all for help
but what if we just add 3 coin as example "etn,xvg,xrp"
and when any user make a deposit , the system automatically convert this coins to btc market price and keep this amount in every user wallet.
until make purchase order.
i think i can do it manually with chat agent, but it will be huge load.
any one can create any system like that, i will be happy to pay for his time
Coinpayments.net already do that, you can accept XRP,ETH and all other altcoins and configure to receive automatically BTC or other coin.
You can set FROM and TO coin in your request.
But if you want use your own system, you have to get the rate of each coin (ETH,XVG,XRP, BTC), using any online API (CoinMarketCap for example).
And then:
Take example of ETH:https://coinmarketcap.com/currencies/ethereum/https://coinmarketcap.com/currencies/bitcoin/1 ETH = 424,34$
1 BTC = 7 685,18$
Let's say you received 3ETH and you want calculate BTC:
BTC = (3 x ETH rate USD / BTC rate USD);
BTC = (3 x 424,34 / 7685,18) BTC;
BTC ~= 0.16564609 BTC
Here is API documentation of CoinMarketCap:
https://pro.coinmarketcap.com/api/v1