Get absolute path of initially run script

I have searched high and low and get a lot of different solutions and variables containing info to get the absolute path. But they seem to work under some conditions and not under others. Is there one silver bullet way to get the absolute path of the executed script in PHP? For me, the script will run from the command line, but, a solution should function just as well if run within Apache etc. **Clarification: The initially executed script, not necessarily the file where the solution is coded.**