Replace variables twice ?

For example i got these codes

So in dS i want the zx and zy be replaced by diff(...) that i typed before And in zx and zy i want to replace them with rcosphi and rsinphi so the highlighted line in the command window will have rcosphi and rsinphi instead of x and y

[Geoff Hayes]

Tran - but aren't you setting x and y to be r*cos(phi) and r*sin(phi) respectively? Or is it just in the output that you want to see this?

[Tran Thien]

Yes But my goal is to solve an intergral in which 'phi' is the variable so I want 'phi's in the highlighted funtion not x and y . I thought if i set x and y like that it will automically replace them with r*cos(phi) and r*Sin(phi) in all my functions

[Geoff Hayes]

Tran's answer moved here

Any help ?