Description:
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From manual page: http://www.php.net/function.mb-substr
---

Documentation states: 

length parameter - If omitted or NULL is passed, extract all characters to the end of the string. See here: http://php.net/manual/en/function.mb-substr.php

Ideally omitting NULL would result in a return value of all characters to the end of the string. However, the next best solution would be to update the documentation so that it accurately describes the behaviour of mb_subtr(). 

I have also noticed that other versions of this page do not contain that misleading statement, such as on docs.php.net


Test script:
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mb_substr('Hello I am a string', 6, NULL, 'UTF-8');

RETURNS 'I'

mb_substr('Hello I am a string', 6);

RETURNS 'I am a string'

Expected result:
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mb_substr('Hello I am a string', 6, NULL, 'UTF-8');

RETURNS 'I am a string'

Actual result:
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mb_substr('Hello I am a string', 6, NULL, 'UTF-8');

RETURNS 'I'