You need to read more, 20th century goes from 1 Jan 1901 to 31 Dec 2000. Source: https://en.wikipedia.org/wiki/20th_century

Thank you :) These little arrow functions come in handy sometimes.

I know about foreach, but that wasn't my mistake.
for is actually faster than foreach, but only if I don't call count() in the condition.

So the faster solution should've been:

$count = count($buses_colors);
for ($i = 0; $i < $count; $i++) {
    ...

This way, count() only gets called once and everything runs faster.
For readability yes, foreach is better.

PS: This entire solution isn't optimized. I might have rushed through this low level kata, as a distraction on a break from work :)