To start external programs from PHP you can use the system() or exec() function. Under Windows this works with the command line PHP (php-cli) without problems.
To execute external programs with the Apache mod_php you have to configure the Apache service. Open the Windows services application (Apache's tray icon has 'open services' or Windows Systemsteuerung/Verwaltung/Dienste), click right on 'Apache2', choose 'Eigenschaften', 'Anmelden' tab and allow 'Datenaustausch zwischen Dienst und Desktop zulassen'.
Here an easy example to open the text file 'test.txt' with the windows standard editor:
<?php
echo exec("start c:/temp/test.txt");
?>For some executables you should use 'start' to prevent PHP to wait on end of execution.