It's not possible to get the whole raw request data from within PHP. You can get the request body using php://input but not the headers. Searching around, I found that you can log the request data from apache using
mod_dumpio. It will dump the incoming request data to error.log. As mentioned in the docs, for apache version < 2.4, you have to set LogLevel to debug. One catch with this is to make sure none of your virtualhost config has LogLevel higher than debug otherwise you'll not get output from this module. Also make sure you didn't set the LogLevel to debug but going down the config, another LogLevel exists and set to something else. Happened to me.