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.

[The following seems to be not necessary, but if you have problems try this:
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 example to open the text file 'test.txt' with the windows standard editor and start Firefox browser:
{{{
<?php
echo system("start c:/temp/test.txt");
echo exec("start firefox");
?>}}}

Note the use of 'start' to prevent PHP to wait on end of execution.