It's actually very simple and "grep" makes it as easy as possible.  I'll outline a few common scenarios and provide some examples.

Say you want to search ALL files within the current directory (and inside/deeper) for the text "phpinfo()"

grep -R "phpinfo()" *

If you want to search only certain files such as only .txt you would do this instead:

grep -R "phpinfo()" *.txt

As you can see this is another very simple but powerful feature of grep to help you find what file contains a certain word/phrase/piece of code etc..