Built-in function using strlen(): The text, "Hello! My name is Kongseng."Contains 27 characters.
The sum of 25 and 50 is 75
The sum of 90 and 100 is 190

Pre-defined functions are functions that are available for use without the need to define/code it.The PHP built-in function that I used as an example is the strlen() function which determines the lenght of the string. User-defined functions are functions that users have created and is defined in the source code unlike built-in functions.