Yea, the question (and it remains so in all of these) is that the first section is at least 1000.

Otherwise it wouldn't really be 8 bytes digits to begin with (not going to argue semantics here).

I kind of assumed that. Otherwise you'll just have to pad both sides and concat or multiply left by 10000, add to right and pad left.

Update: You could just

sprintf( "%08d", $var1*10000+$var2 )

and call it a day, but that's actually slower than any of the others :(