[PR][php-langspec][#172][master] - `exit` with no parenthesis does have an opt. expr.

You can view the Pull Request on github:
https://github.com/php/php-langspec/pull/172

Pull Request Description:
`exit` is valid, `exit(42)` is valid, but `exit 42` is invalid. The optional expression must be surrounded by parenthesis.

See https://github.com/tagua-vm/parser/pull/73#issuecomment-259902148.