PHP Multibyte String often referred to as MBstring may not be enabled in many common php configurations, but may be needed in certain WordPress configurations, dependent on certain plugin use.
We ran into this after upgrading to WordPress 4.5. Our self administrated server did not have this php module installed. This article shows how to enable this module in WHM / cPanel servers using easy Apache.
From php.net: When you manipulate (trim, split, splice, etc.) strings encoded in a multibyte encoding, you need to use special functions since two or more consecutive bytes may represent a single character in such encoding schemes. Otherwise, if you apply a non-multibyte-aware string function to the string, it probably fails to detect the beginning or ending of the multibyte character and ends up with a corrupted garbage string that most likely loses its original meaning.
mbstring provides multibyte specific string functions that help you deal with multibyte encodings in PHP. In addition to that, mbstring handles character encoding conversion between the possible encoding pairs. mbstring is designed to handle Unicode-based encodings such as UTF-8 and UCS-2 and many single-byte encodings for convenience.
1. You can access EasyApache by going to the Software section of WHM and clicking on EasyApache. Command-line folks can just use /scripts/easyapache.
2. If you’re asked to update, do so then come back to this screen. Updates often contain enhancements to allow EasyApache to support updated technologies.
3. On the first screen (“1. Profile”), select “Previously Saved Config (** DEFAULT **)” and then click on “Start customizing based on profile.”
This setting will display your current settings throughout the rest of the EasyApache interface. This way you don’t need to lookup what your current configuration is, you just change the items you want to have changed (e.g. adding mbstring support to PHP)
4. Continue clicking “Next” until you get to “5. Short Options List” then click “Exhaustive Options List” at the bottom of the page.
5. Under PHP, check the checkbox for “Mbstring” then click “Save and Build”
6. Follow any further on-screen instructions that appear.