PHP class that behaves like enums
I would simulate the enum using class constants.

class VoucherType {
  const INVOICE = 0;
  const CREDIT_NOTE = 1;
  const ADVANCE = 2;
}

$myType = VoucherType::INVOICE;


They are accesible from everywhere you can access the class, virtually everywhere if you use (and you should) an autoloader.

This is a readable and well-known pattern but there are more sophisticated options, starting with SplEnum and ending with php7-enum, but I like the simplicity of the constants. 

Also I you are new to PHP I recommend you to read other's code to learn and get used to the language.