Unlink deletes a file, unset destroys a variable. In large web applications (particularly server applications that run 24/7), you want to frequently destroy variables you don't need so as to ensure there is no memory leak.
no offense waterbomm, but the php manual is the best book on php; and this q was answerable in a couple of seconds of reading.. as a helpful tip - the php site has an autofind feature so you can simply type in http://php.net/fuction-here (eg: http://php.net/unset) and it'll jump to the page
yeap. you right! php manual is the best book and the best teacher for all people learn about php. thanks.