Fatal error: Call to undefined function mb_eregi(). What should i do?

mb_eregi is a standard PHP function, which is supposed to be in all PHP versions from 5 and on.

Looks like your hosting provider has disabled the mb_regex library and mbstring php in your server. They're not supposed to do that.

You can ask their support, referencing the error message and ask them to enable this in your account.

I'm pretty sure that the hosting company will know what this means.