Check version in .php-version file

For security reason, we should check that content in .php-version is a version number. 

For example in astral-sh/uv: 
- https://github.com/astral-sh/uv/blob/bee2baa64e37401367ffd32f81a33e2d26e68f48/crates/uv-python/src/version_files.rs#L116
- https://github.com/astral-sh/uv/blob/main/crates/uv-python/src/discovery.rs#L39