PHP compiler/interpreter
Just run
sudo apt-get install php5-cli
To run it in interactive mode:
php -a
Run a file:
php /path/to/file
Run the interpreter (there's no compiler in default PHP) using the "php" command, yes. If you'd like to know where "php" comes from (what executable is launched), type
which php
This is likely to give you "/usr/bin/php" as an answer, unless you have a special setup.