如何在php中将公共api设为私有

Question

we have created api for our web application, and we want to make it private, which means no one other than us should be able to use it, but right now that api is accessible to public which we want to control, we saw how facebook restrict to application access to the given domain, how can we do that, so only certain domain can access those api calls and others will be rejected? we are using php as our backend, since we are making post request, we cannot pass secret keys which won't work as expected as anyone can use to keys