Description:
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Before PHP 5.4.18 and 5.5.1 a null or 0 value in the third argument returns the whole string from $start when $start is positive.

Test script:
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$value = 'ˆCODE128:Š';
var_dump(grapheme_substr($value, 1));
var_dump(grapheme_substr($value, 1, null));
var_dump(grapheme_substr($value, 1, 0));

Expected result:
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string(10) "CODE128:Š"
string(10) "CODE128:Š"
string(10) "CODE128:Š"

Actual result:
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string(10) "CODE128:Š"
string(0) ""
string(0) ""