That looks pretty cool, did a test (10,000 times in a loop):
rand();
Result 1: Page generated in 1.496 seconds.
Result 2: Page generated in 1.400 seconds.
Result 3: Page generated in 1.486 seconds.

array_rand();
Result 1: Page generated in 0.087 seconds.
Result 2: Page generated in 0.078 seconds.
Result 3: Page generated in 0.076 seconds.

Didn't realise the results would vary so much.