Physical changes are those where new substances are not formed and the changes are, generally, visible (or can be easily observed). Boiling and melting are example of physical changes. In each of these examples, even though molecular rearrangement may take place, the identity of the substance does not change.
Chemical changes are characterized by formation of new chemical species. Reaction of hydrogen and oxygen to generate water is a chemical change. So is rusting of iron.
Photosynthesis is the process through which plants convert light energy into chemical energy. Here is the chemical reaction involved:
`12 H_2O + 6 CO_2 + Light -> C_6H_(12)O_6 + 6O_2 + 6H_2O`
As we can see, water and carbon dioxide combine to form glucose and oxygen. Since new chemical species are formed, photosynthesis is clearly a chemical change.
Hope this helps.