Could you please elaborate where and why your $combined2 is better?

(UPDATE: If there is a quantifiable reason why "X" should be ordered before "A", I'm sure this can be added to the sort function, see cmp_more_frequently($a,$b) )

It wasn't really a requirement, other than that every time you give the algorithm those inputs in that order, you get the same output. In other words I don't want it to depend on hash randomization, so I think original input order should be used as a tie-breaker. When I built the example, I put 'A' after everything else since there was no relation to the previous rules and the rule with 'A' came later. Same with 'Z' vs. 'K'.