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Setting up HTTP/2 on the Apache HTTP Server with PHP compatibility

Reconciled

Article from ADMIN 76/2023

By Eugenia Bahit

If you are running PHP applications, setting up HTTP/2 on the Apache HTTP Server can be a bit confusing because of some incompatibilities between the Apache HTTP/2 module and the Prefork multiprocessing module.

HTTP/2 is the second version of the hypertext transfer protocol and brings two significant improvements: more efficient network resource usage and reduced latency.

Latency (the elapsed time between the user sending a request to the server and receiving the response) can either increase or decrease according to the way network resources are handled, so higher latency means a longer wait for the user.

To reduce latency, HTTP/1.1 implemented a request pipelining technique, allowing TCP connections to send multiple requests without waiting for a response. However, although request pipelining facilitates concurrency, it does not avoid head-of-line (HoL) blocking, which occurs when a single data packet queue blocks subsequent transmissions while waiting for a response. This phenomenon affects both the TCP and HTTP protocols, so it appears at both the Transport and Application layers of the TCP/IP protocol stack.

HTTP/2 solves the HoL blocking issue for HTTP at the Application layer, improving concurrency and reducing latency. For that reason, setting up HTTP/2 will enhance the performance of the websites running on your server.

If your website uses PHP, however, you'll need to go to a little more effort. The HTTP/2 protocol on the Apache HTTP Server requires a conventional multiprocessing module (MPM) such as MPM Event. The problem is that the default PHP binary for Apache installs MPM Prefork as a dependency, which is not compatible with the HTTP/2 protocol.

A straightforward approach to solving the PHP compatibility issue is to replace the default PHP module with FastCGI and configure Apache to work with MPM Event.

The following guide is tailored to Debian 11 and PHP 7.4, but you can adapt it by applying minor changes, which you can find in the corresponding steps.