Remove executable bit from all PHP files

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | 
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

I've noticed a non-trivial number of pull requests to remove executable bits. Could there be an automated test to prevent files getting into the repo with them on in the first place?