How to move an array value to the beginning of the array?

I have a PHP array similar to this:

0 => "red",
1 => "green",
2 => "blue",
3 => "yellow"

I want to move yellow to index 0.

How do I move any one of these elements to the beginning? How would I move green to index 0, or blue to index 0? This question is not exclusively about moving the last element to the beginning.