[ticket/11325] Check PHP 5.6 compatibility

with php 5.6.0a3 the default configuration of mbstring changed from
'mbstring.http_input=pass' to 'mbstring.http_input=' so the new default
configuration use the value of 'output_encoding' which is empty by default
and so use the value of 'default_charset' which is 'UTF-8'.

PHPBB3-11325