[PHP, PHP7] sys.FileSystem.exists inconsistent with Neko when path to a file has trailing slash

If an actual file `/tmp/some.file` (not directory) exists and following code is run
```
trace(sys.FileSystem.exists("/tmp/some.file")); // Neko: true, PHP/PHP7: true
trace(sys.FileSystem.exists("/tmp/some.file/")); // Neko: true, PHP/PHP7: false
```
thus requiring to use `sys.io.Path.normalize` before using this function on PHP target.

Don't know if it should be treated as a bug but this inconsistence between targets should at least be mentioned in the docs (probably as well as a fact that calls to that function are cached by PHP).

Didn't check with other targets (seems specific to PHP though).