Small simplification

[nazar-pc]

Original code seems over-engineered

[Ocramius]

Was this just dropped? Was there no coverage for it?

[nazar-pc]

If you analyze it correctly - it basically does nothing.
First we cut Content- at the beginning, then add it unconditionally (why?). After this we check whether remainder is equal to MD5 and if so - pass it directly, otherwise we make reminder lowercase and than make first character uppercase. After all manipulations we make it lowercase (which basically neglects everything from previous sentence).
So:

• cutting CONTENT_ and replacing with Content- seems reasonable, but can be just replaced with strtr() (the real difference in practice is just one symbol) in order to keep is homogeneous with previous block of code
• all the game with repeated upper/lowercasing doesn't make any sense

As far as I saw there are test cases for all situations touched by these changes.

[weierophinney]

I'm going to keep the code as-is, for the following reasons:

• Having two separate blocks makes it quite clear what the two header sources are within $_SERVER. I personally feel this is important for developers trying to understand how $_SERVER works within the standard SAPI implementations.
• Lower cyclomatic complexity.

[nazar-pc]

I can refactor it to have 2 separate blocks, but leaving as-is means by having lower cyclomatic complexity there will be much more CPU cycles wasted for nothing

[weierophinney]

there will be much more CPU cycles wasted for nothing

This code runs at the start of a request, and will be executed for only as many entries as are present in $_SERVER. The changes are a micro-optimization at best, and sacrifice clarity.

[nazar-pc]

Final change from my side, refactored as promised: nazar-pc@c3cc4b4
If you really really think that clarity was sacrificed, then I have nothing to add.