A built in function is one that is standard and pre-built in PHP code. The user does not write or modify it.
A user defined function is one that the user creates by writing the code and using () behind the word. This kind of code is very useful, for example I could set the if/else code from that page as a function and call it without typing all the code again. In the below example, I call the names of my pets in this simple function.

What a useful pre-built. THIS IS A PREBUILT FUNCTION TO CHANGE THE STRING TO ALL UPPERCASE LETTERS.

This will call the names of my pets.
Mishka
Mink
Percy
Phillip
Phobos