What is the use of library functions in PHP?

[tamil96]

Hi..
I am Tamil. I am going to create a website for my business. I have a doubt that what is the use of library functions in PHP language.

[Weedpacket]

Saves you having to write the functions yourself over and over again.

[dalecosp]

Forgotten already? http://board.phpbuilder.com/showthread.php?10395999-What-is-the-use-of-library-files-in-Web-development&highlight=library