Functions that are built-in into php to perform some standard operations.
select one:
a. user defined functions
b. predefined functions
c. undefined functions
d. functions

(b)  Predefined Functions.

Explanation:

Predefined functions are those functions which are already present in the library of the language to perform some standard operations we need not to define these functions we can use them directly.Some examples of PHP buil in functions or pre defined functions are as following:-

imagecreatetruecolor(),mysqli_connect(),phpinfo() etc.

i believe the two important functions that skype uses are nat traversal and user location.good luck        

explanation: