You'll probably need to look at installing php-cgi to run php scripts from the command line. For debian, simply apt-get install php4-cgi. Then you simply call:
Include that code in quick shell script such as:
echo "Running PHP script on boot..."
save it to myphp.sh or whatever you want to call it within /etc/init.d and give it execute permissions:
chmod a+x /etc/init.d/myphp.sh
then symlink to whatever run level you want it to start at:
ln -s /etc/rc2.d/S30MyPHP /etc/init.d/myphp.sh
to start at runlevel 2, for example.
What is you're trying to actually do with your PHP script?