Description:
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strtr() only translates _bytes_, not characters.
When translating a multi-byte character to a single byte character, only the first byte is converted, resulting in a broken string.
The examples given in the doc work with an ISO-8859-1 encoding of the source file, but not with UTF-8.

IMHO, two solutions;
1) Fix the function. But then the function would need to guess the file encoding. Hmmm....PHP6?
2) Fix the doc.
   The doc title, "strtr — Translate certain characters", is misleading. Either replace "characters" by "bytes", or add somewhere on the page that it only works on single-byte encodings. A big warning would be useful.

Test script:
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// in an UTF-8 file
echo strtr("’", "’", "'");
echo strtr("äåö", "äåö", "aao"); // example from php.net

Expected result:
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'aao

Actual result:
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'��oao�o�