About HTTPS, what to do with

About HTTPS
Recently encountered a new requirement, is about the HTTPS interface, the target interface is HTTPS, the client uses PHP development, I do not know the knowledge of HTTPS, so ask you:
1. What software do I need to install if I use HTTPS on the server side? (Web server is Apache)
2. What software does the client need?
3. Why do some HTTPS websites need to install the client certificate, some do not need?
4. How do I initiate a GET request for HTTPS using PHP?
------Solution--------------------
1 no software required, but Apache needs to compile or link the OpenSSL module
2 browser
3 need to verify client identity
4 curl, a parameter is a certificate. But note that PHP also needs to compile or link OpenSSL at this time.

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