Getting blockchain api key is now hard these days so i used block.io before and also coded bitcoin receiving platform for few of my clients. You can find their api documentation here
https://block.io/api/simple/phpbut their limitation for free package is not useful if you need to generate huge number of addresses a day.
Instead you can try blocktrail api, however i haven't worked on their api but they seem to provide so much even for free package.