Developer's Guide : Backend
Backend Guide: Pages
introduction
integrating CPAINT
implementation differences
generating XML
complex return-data
arbitrary character encodings
PHP Implementation Differences
By default, all local PHP classes and variables are not available to functions unless they are "imported" using the PHP global language construct. Therefore, at the beginning of the function, you need to import the instance of the CPAINT backend class.
function calculate_tax($sales_amount) {
global $cp;
return($sales_amount * 0.075);
}