You need to modify you startup script for your apache.
Open your startup script, it should be in /etc/init.d/apache or apache2
Search for the start / restart section and add your cli call for your PHP script.
restart) [..] php -q /tmp/myscript.php & ;;
Where /tmp/myscript.php is your php script that you want to launch.
The "&" at the end will start the script in the background so your startup will not wait until your php script has ended. If you want to wait until it has ended, remove the &.
You should not put such thing into your startup scripts, there might be better solutions. What are trying to achieve?