If you have had any encounter with a WordPress theme, WordPress plugins etc the first thing you would require is, knowledge of PHP. I think PHP is one of those languages that everyone aspiring to have (or already having) a web presence should learn.  Here is how you can install(its a bit tricky) and start learning PHP.

Installation:

All steps given below are for Windows XP SP2

1. PHP is a server side scripting language. To work on this language, you need to have a web server running successfully on your local machine. Though PHP goes well with many web servers, for our learning sake we will install Apache on Windows XP.
2. Go To http://httpd.apache.org/download.cgi
3. Download the latest version of Win32 Binary without crypto (no mod_ssl) (MSI Installer) [As of writing this post Apache HTTP Server 2.2.9 is the best available version]
4. Now follow each and every instruction carefully at this link http://www.buzzle.com/articles/109196.html
5. By now, Apache must be up and running successfully.
6. Now we will install PHP. Go To http://www.php.net/downloads.php
7. Under Windows Binaries, download the PHP 5.2.6 zip package[Current as of writing this post]
8. Again, follow the instructions religiously given at this link http://www.buzzle.com/articles/109200.html
9. Now, if after following all the steps given at link of pt 8 above, you encounter an error such as "httpd: Syntax error on line 486 of C:/server/Apache2.2/conf/httpd.conf: Cannot load C:/server/PHP/php5apache2.dll into server: The specified module could not be found", go to pt 10 in that file change the first line from c:/server/PHP/php5apache2.dll" to c:/server/PHP/php5apache2_2.dll" Hopefully that should solve the problem.
10. Test if PHP is working as given in pt 16 of link above.   

Learning PHP

Now that’s simple. Go through this link. It is a very easy step by step guide for anyone wanting to learn PHP. Though the articles on this link were written way back in 2004, I find that nothing has changed till now.

Do comment about your experience of learning PHP.

How to install and learn PHP? is a post from ReviewOfWeb