This is an example of the strtoupper Built-in function which turns every character in a string to uppercase:
HELLO, MY NAME IS ALEX!

This is an example of a User-defined function which multiplies two numbers stored in variables that I declared:
10 multiplied by 5 is 50

Built-in functions perform common tasks and are already built-into PHP. User-defined functions are created by the user to perform a custom operation.