Extend classes without multi-inheritance or duplicate code

I'm trying to figure out how to correctly extened several classes without duplicate code. I have the following classes:

• Base (method A, method B)
• Invoice : Base (method C)
• Shipment : Base (Method D)

I have to adjust some functionality without modifying them. The problem is that some methods I need to adjust are in the base class.

If I extend the Base class, I will need to copy Methods C and D. If I extend each of the final classes, then I need to duplicate the changes to methods A, B.

How do I correctly do this without multi-inheritance (PHP, not possible) or duplicate code?