Syntax: call_function (STR func-name, arg, ...) => value
Calls the built-in function named `func-name', passing the given arguments, and returns whatever that function returns. Raises E_INVARG if func-name is not recognized as the name of a known built-in function. This allows you to compute the name of the function to call and, in particular, allows you to write a call to a built-in function that may or may not exist in the particular version of the server you're using.