Built-in function example
The name 'Kaila' is 5 characters long.
User-defined function example
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PHP has over 1000+ built-in functions. The built-in function below takes a string as an input and returns its length as an number/integer. In the first example, the length of the string should be 5. Meanwhile, The user defined function is a reusable function that is created by the user in order to complete or satisfy a specific requirement for a program.