Support PHP8

[brainexe]

there no BC visible to me :)

[Jeroeny]

Anything holding back this merge? :)

[ktomk]

On Travis-CI there is yet no full support for php: 8.0 - which is why this PR build still appears currently red to me - , PHP 8 support could work by nightly/8.0snapshot and/or using the standard Linux OS/dist (Xenial).

So for this PR if I may suggest: add an allow_failures job for php: 8.0 (the recently added one; and keep everything else).

jobs:
  fast_finish: true
  allow_failures:
    - php: '8.0' # php 8.0 missing on travis <https://travis-ci.community/t/php-8-0-missing/10132>

And then test again without specifying trusty - if green, remove the allowed failure. See as well the link in the example above.

[ktomk]

@brainexe Thanks for taken care, I cherry picked the applicable changes in a different PR and closed this one. Thanks as well @Jeroeny.