Why in PHP if you use Mkdir recursive flag do the nest directories not chmod?

Why in PHP if you use Mkdir recursive flag do the nest directories not chmod?

I am using mkdir to create normally 2 nested directories for a file
structure. The directories it creates are always set to 0755. The code I
am using however is.
mkdir('path_one/path_two', 0777, true);
I have tried then doing
chmod('path_one/path_two', 0777);
but that only sets the final directory as 0777. What would cause mkdir not
to function properly?