PHP missing mb_strpos function?

I would guess that your version of PHP wasn't built with the "--enable-mbstring" option.

You can check with the phpinfo() function. There should be a "mbstring" section. Mine has this:

mbstring

Multibyte Support   enabled
Multibyte string engine libmbfl
HTTP input encoding translation disabled

MBString is not a default extension in PHP and you have to install it if you are running linux try

yum install  php-mbstring.x86_64 # for RedHat derivative systems

as a root user or

apt-get install php-mbstring.x86_64 # for Debian derivative systems

on windows you have to edit php.ini to use the extension dll library

Note: mb_strpos still supported in PHP 5

sounds like it's simply not installed. please take a look at the documentation:

mbstring is a non-default extension. This means it is not enabled by default. You must explicitly enable the module with the configure option. [...]