If you're a PHP programmer, you should definitely know Linux. PHP was created on Linux, and PHP is an imporant part of the LAMP stack: Linux+Apache+MySQL+PHP. Even if you have the opportunity to develop for Windows servers now, your next job will probably have you developing for Linux servers, and you'll need to be familiar with the Linux environment so that you're not running afoul of Window specific 'isms of PHP.
In the mobile app space, knowing Linux will (sadly) have little relevance to the environments you're developing for. iPhone OS has nothing to do with Linux, and Android has you so tightly fenced into a virtual machine, you'll never know there's Linux beneath you. You could program for Linux on Meego or Maemo, but those have nowhere near the popularity of the iPhone and Android.
In the embedded space, you have to know Linux. Embedded Windows is rare, but embedded Linux is quite common. (Of course, bare-metal embedded programming is also quite common, so it depends where you want to go.)
If you learn Linux in depth over a number of years, you'll have some level of understanding of every part of the system from the kernel, through the bootup process, up to the command-line, and GUI.