[Process] Improve the usability of the PhpExecutableFinder

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

I first wanted to make `find` static but that would probably be considered as an unrequired BC break.