Linux For PHP Programmers
If you plan to take your PHP / mySQL programming seriously, you will more than likely end up using the LAMP stack. LAMP stands for (Linux, Apache, PHP and MYSQL). Initially, you will need to use php and mySQL to make a database driven website. However, you should be very aware that these technologies are installed on Linux. If you use shared hosting, you will not have shell access and be able to alter and navigate the Linux file system. But, as your website(s) grow, you will eventually need a VPS or dedicated server. With a VPS or dedicated server comes responsibility. Here is where the more Linux you know, the better off you are. With knowing some Linux through practice, studying or certification, you will have much more control to knowing what is happening. In addition, Linux has various other programming languages for which you can write scripts to monitor your file system, or, add into your php scripts. Essentially, you can use shell commands, sed and awk in your php (if you really want to).
Linux Home Server
Asides from using a data center to host your websites, your Linux knowledge can extend into home usage. Linux variants like Ubuntu and Centos make great operating systems at home. In fact, you may find yourself spending more time on Ubuntu than Windows 7. Why? Linux can run much faster, boot much faster and you can create a home server to test files and even host a simple, lower traffic website. If you are familiar with Wamp (Windows, Apache, mySQL, and PHP) you may want to explore LAMP at home. Personally, I have found Lamp to be a much better experience to use than Wamp; especially using curl, pear extensions and adding custom extensions and software. Also, using Lamp at home will accelerate your knowledge for the Lamp you use with a VPS or dedicated server.