You can use apache as a webserver. Php then can be run as either a stand alone executable or as a module in apache. Apache is available in Win95,98,NT.
In order to learn PHP, you will need a webserver. But, the webserver does not have to be made public and just runs in the background on your computer.
I learned a lot of PHP by installing Apache and PHP4 on my laptop. Worked great! In fact, I still use it to trouble shoot scripts when I don't have FTP access (i.e. when on the road). Best of luck