If you’re getting a fatal error notification that begins with the following description “Uncaught Error: Call to undefined function ctype_xdigit()” , that means that the ctype_xdigit function is missing from PHP version installed on your hosting.
These functions are used to check whether a character or string falls into a certain character class. You can find out more about these functions here.
Usually, the ctype_xdigit function comes bundled with other ctype functions as a part of the default PHP installation. Some hosting companies might leave the ctype_xdigit function out.
How to resolve this?
As mentioned, this is related to the functions included in PHP version your host installed on your server. Thus, the solution for this is contacting your hosting provider and asking the host to re-/enable the ctype_xdigit function for your website.