If you've never tried it before, automated testing can be a somewhat bewildering step, but it is one that makes your applications far more dependable and maintainable.
Test-driven Development (TDD) is one of the most common paradigms. Here, you write your tests before your code, make sure they fail first, and then write code to make them pass. Once this is done, you "refactor", cleaning up and reworking your code but ensuring the tests still passes.
Note
For a helpful general introduction to Test-driven Development, refer to http://code.tutsplus.com/tutorials/the-newbies-guide-to-test-driven-development--net-13835.
Now, let's look at the recommended testing libraries for the different languages and frameworks we've mentioned:
For PHP developers, Tuts+ has a series of guides on TDD using PHP; refer to http://code.tutsplus.com/series/test-driven-php--net-27482 to get started.
For Ruby developers, the RSpec testing framework is a great choice. Code School has a helpful course at...