mb_ series functions have errors

[wujwmail]

Issues discovered through the experimental project https://github.com/wujwmail/PHPExcel_test:

The mb_ series functions have errors when handling UTF-16LE and UTF-16BE character encodings! For example:

<?php

  $utf8_str="这是一个测试";

  $iconv_utf16=iconv('UTF-8', 'UTF-16LE', $utf8_str);

  echo "iconv_to_UTF-16: ".base64_encode($iconv_utf16).PHP_EOL;

  $mb_utf16 = mb_convert_encoding($utf8_str, 'UTF-16LE','UTF-8');

  echo "mb_xx_to_UTF-16: ".base64_encode($mb_utf16).PHP_EOL;

Native PHP execution results:

      iconv_to_UTF-16: 2Y8vZgBOKk5LbdWL
      mb_xx_to_UTF-16: 2Y8vZgBOKk5LbdWL

Peachpie execution results:

      iconv_to_UTF-16: 2Y8vZgBOKk5LbdWL
      mb_xx_to_UTF-16: 6L+Z5piv5LiA5Liq5rWL6K+V