PHP Number to Text Converter: Encode and decode numbers using character mappings

This class can encode and decode numbers using character mappings.

It can take a given number and encode it using a given list of characters to map the number.

The numbers may be encoded using pre-defined characters sets like upper or lower case letters, digits including 0 or not, and underscore or dash symbols. A custom list of characters may also be included.

The class can also do the opposite, i.e. take a previously encoded number with given character sets and decode the original number.

The class uses arbitrary precision math to allow for very long numbers to be converted.