What is += used for?

It's adding all those values to time.

something += somethingelse

is a shortcut for

something = something + somethingelse

-Adam

$time += $diff['hours'];

is the same as saying

$time = $time + $diff['hours'];

a += 2; is the equivalent of a = a + 2;

At one time with some languages (especially very old C compilers), the compiler produced better code with the first option. It sticks around now because it's a common idiom and people used to it think it's clearer.