[REQ][PHP] Add minimal CI config to PHP generators

Is your feature request related to a problem? Please describe.

Recently noticed how many PHP based projects migrates from Travis-CI to GitHub Actions. I didn't try it yet, but after a quick reading I see that GitHub Actions has Ubuntu environment with preinstalled PHP from 7.4 to 8.1, while Travis CI offers Bionic Env with PHP 7.4 only. It can speed up tests significantly I guess.

Since I never used other CI tools but Travis CI, I would ask our community to share their practices regarding PHP CI. If somebody can drop a config to test generated samples under PHP 7.4, 8.0 and 8.1 via GitHub hosted runners I would much appreciate that.

Describe the solution you'd like

• Remove Travis CI config from current repo and from all PHP based generators samples.
• Setup GitHub Actions to autotest PHP based generators also add basic config to samples.

Describe alternatives you've considered

Suggest other tool if GitHub Actions doesn't fit.

Additional context

This might be a good first issue for new contributors since you don't need to know Java.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon
active PHP contributor @Articus

Start by locating the Travis CI configuration in the repository and the PHP-based generator samples. Review how GitHub Actions can test generated samples on PHP 7.4, 8.0, and 8.1, then verify that the repository and samples use the new CI configuration and no longer depend on Travis CI.