Another simple feature that’s good to pick up on!
When you want to run the same functions from several pages, but don’t want to copy/paste (because that’s BAD), you do this.
Usually, you’d use require or include to include the code from one PHP script in another.
If you hadn’t done this before, or designed it in, you’ll probably need to extract the functions from your script(s), place them in a central file, and include that in all scripts that need it (including, of course, the one they were originally in!). Remember that includeing a file in another causes its embedded HTML to be included too, so you’ll typically include PHP files that have functions only, to achieve the desired result.
Then you just call the function, as if it were written in the script in the first place 🙂