Support old PHP versions

[phfroidmont]

trafficstars

As suggested in #240, adds support for all stable php versions from https://github.com/fossar/nix-phps

[DavHau]

Thanks for that. According to the readme of nix-phps: There is also a php package which is the alias of the default PHP version in Nixpkgs. Can we ensure that the nixpkgs php remains the preferred choice over the other php packages?

@tinybeachthor pinging you in case you'd like tor review this.

[phfroidmont]

php being first on the list it will be the first considered if I'm not mistaken. Still, I can add a bit of logic to ensure it is always considered first, no matter the ordering of the list.

[aszenz]

Will php still work with the V1 api?