Gets the specified argument from a user-defined function's argument list.
This function may be used in conjunction with func_get_args() and func_num_args() to allow user-defined functions to accept variable-length argument lists.
Generates a warning if called from outside of a user-defined function, or
if arg_num
is greater than the number of arguments
actually passed.
Note: Because this function depends on the current scope to determine parameter details, it cannot be used as a function parameter. If you must pass this value, assign the results to a variable, and pass the variable.
Note: This function returns a copy of the passed arguments only, and does not account for default (non-passed) arguments.