array_unshift() prepends passed elements to the front of the array. Note that the list of elements is prepended as a whole, so that the prepended elements stay in the same order.
Returns the new number of elements in the array.
This would result in $queue having 5 elements: "p4", "p5", "p6", "p1", and "p3".
See also array_shift(), array_push(), and array_pop().