Can php auto execute it's script in schedule without opening a webpage?

Dear all I just wonder did anyone know if php can act as a scheduler rather than execute script manually? i want to set a schedule for php to run certain script at specify time, to what i understood in php is : the script can only be process when a homepage had been execute. but i want the script to be excute even no one open a homepage contain php script in it! is there anyway i can do that? thx Jack