Very basically - putting any additional commands in your scripts will increase the server load, because its one more thing the server has to do. If you're worrying whether or not you can create your own functions: You can do it. Creating a function is faster than putting the same code 10 times throughout your code.
It depends on the way you use the function.if the code needs to be excuted only once means it is waste. if the code should be executed more times then it is recommended to use functions than repeating the code.
If you include the function you aren't really increasing any loading time but the amount of memory used which in turn can slow down other functions that are being used.
I am using wordpress and i need to call to certain functions within a LOOP more than 10 or 15 times. So i am just worrying about server load and other related.
You should worry more about DB queries and functions that parse large amounts of data (and cache them, of course). Small functions really make no big difference. You can read more about optimizing your PHP code on blogs like this: http://www.chazzuka.com/blog/?p=58