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php-src / unicode-gotchas.txt

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* md5() on Unicode string will obviously be different from md5() done on
  a native encoding version of that string. This can trip up people who
  generate signatures externally (storing them in DB for instance) and then
  pass md5() of Unicode strings to check against that. The solution is to
  always run md5() on a string of the same type and encoding that the
  external md5() was run on.