when I use toUpperCase and toLocalUpperCase function,why do they have no effect ?

a example(2,263,140)
Expected objects to be the same.
Passed in:
(string) '02FF8C'
Expected:
(string) '2FF8C'

but why the result of example(0,0,0) is '000000' not '00000'

I use the function array_push(),get reslut like this:Expected: Array (0 => 'arp' 1 => 'live' 2 => 'strong')
Actual : Array (0 => 'live' 1 => 'strong' 2 => 'arp')
however,I also use array_unshift() and array_reverse() which get the same result.
how does it work?