Description:
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From manual page: http://www.php.net/function.mb-substr
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Check this:
http://3v4l.org/e1Pc4

1) The behavior of mb_substr with a negative value as first parameter is completely undocumented. (except for a user's comment)

As expected, it matches that of substr, using the negative value as a starting character counting from the end of the string, but it needs to be documented.


2) The behavior with a negative value as the first parameter and NULL as the second changes at version 5.4.8
Starting from that version, it behaves as expected (taking the string to its end); previous to that version it returns an empty string.
The behavior in older versions was obviously a bug, but the change should be documented.

Also, if there's a value to be used instead of NULL as a workaround, that needs to be documented too.