Description:
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When php parses the source files and converts it to bytecode, does it do dead code elimination? For things like

if (false) {}
or when using constants (self defined or predefined)
or when checking certain expressions such as   if (4 > 10-15) {}

I couldn't find anything about this after searching. It would certainly benefit libraries which have a lot of version checks and checks on which operating system it's using. To me (unknowingly of anything happening in the compiler) it seems a bit low hanging fruit optimization, but perhaps to cover all the edge cases is not so trivial. However a partial implementation would already help a lot. In case it does exist please link to relevant documentation or source files.