PHP Get random element out of array 1, show, and move to array 2

Ok i'm trying to make a page that when i press a button randomly returns an element out of array1, saves it (so i can show it on the page), and then moves it to array2 so the element doesnt get returned a second time, but i have no idea where to start.

$subject = array (
array("Title1","comment1"),
array("Title2","comment2"),
array("Title3","comment3"),
array("Title4","comment4"),
);

help would be enormously appriciated!