To let opcode caches optimize cached code, the `PHP_VERSION_ID`
constant is used to detect the current PHP version instead of calling
`version_compare()` with `PHP_VERSION`.
If the response contents has been sent before an error occurs, PHP
triggers the warning "Cannot modify header information - headers already sent"
This change ensure that the error message is echoed, while it's impossible
to change the HTTP status code and headers.
