Deprecated. Don't use for new development and try to migrate existing uses.
This function takes an integer string and generates the check digit to append to the end of it in order to make the string pass a modulus 10 integrity check.
This is the kind of integrity check used for things such as social security numbers and OCR-numbers.
If the input string is either null or if it contains any character that is not a digit then the function will return a value of null.
The generated check digit.