Add PHP 7.4 checker

[kazazor]

PHP 7.4 was released on November 28th, 2019 - GA release.
It has some incompatible changes.

It would be great to add it.

[kazazor]

Seems like work started on #251

[NetScr1be]

Commenting to bump. Same issue.

[phxdnelson]

This would be great. I see 7.4.9 is the current stable release since August 6, 2020 and 8.0 is coming fast.

I don't see any activity in #251 in a very long time.

[Thomas-1985]

I would really like to see this too