Improve mb_substr using native substr.

Signed-off-by: Hugues Peccatte <hugues.peccatte@gmail.com>
Minor PHPDoc modification.

Signed-off-by: Hugues Peccatte <hugues.peccatte@gmail.com>
Replace uses of string iterators by use of mb_substr.

Signed-off-by: Hugues Peccatte <hugues.peccatte@gmail.com>