The code is best viewed with a display width of 1280 or without text wrapping.  
Running on the command line, it evaluates reverse polish expressions (entered by the user at runtime) 
with spaces between all operands and operators (for example: 10 5 + 5 / 3 4 * +).

The program returns:

(value of expression):kinyw
?ceinok

at this point you can either enter another expression or "ceinok" ('end' in reversed polish) 
to end the program.

Requires the PHP interpreter and the PHP cli.