I have long been wishing to write down which heavy trafficed and popular sites that utilize PHP as their scripting language. Now as there right now is an ongoing effort to create a white paper about the business case for php and why to use php. I felt this would be the right moment to write down the ones I know and check up on the alexa top 500.

1. Yahoo INC
   Huge web portal / The top dog in traffic on the internet beating both Google and MySpace.
   
   

2. Wikipedia.org
   The open online encyclopedia.
   
   

3. Friendster.com
   Online social network
   
   

4. Facebook.com
   Another online social network, recently rejected a billion dollar offer from Yahoo.
   

5. Digg.com
   Social news network.
   

6. Sourceforge.org
   The world's largest Open Source software development web site.
   

7. Flickr.com
   Online photo sharing and storing: Actually this web site is now part of Yahoo Inc, but it did not use to be and as a standalone it has been pretty impressive, and it is still listed independently on Alexa's top 500.
   

Now on Alexa's top 500 I stumbled upon a few other big PHP photo sharing and storing web sites that runs PHP: http://www.imageshack.us , http://www.photobucket.com , http://www.imagevenue.com .

And finally of course I would like to point out the widely popular Wordpress ( http://wordpress.org/ ): An open source blog platform.

Now the last big question is of course how do you know that these sites utilize PHP. Well there are a few different ways:

The first one is to look at the file extensions in the urls on the web site, if you see the file extension .php you can be pretty sure the web site is utilizing PHP.

The second is a bit tricky, a well hidden public secret is the PHP easter eggs , if you for instance go to: http://www.sourceforge.org/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000 you will see a credit page for the PHP core developers! Then the site also utilize PHP. Combined with this you can also look at the http-headers the server use to respond using the firefox plugin live http headers: if you see following header “X-Powered-By: PHP/5.1.6” or something that looks a lot like that then the site also utilize PHP.

Finally the third way - the non technical: If a company hires PHP developers it is also a very clear indication that they utilize PHP.

If you think I missed out something in this list, please let me know!