My understanding is that chloroquine phosphate and chloroquine diphosphate are the same thing in terms of application. If you look at Sigma Aldrich's website, you'll find that both compounds have the same formula and molecular weight. Not sure why they have two discrete names, though.
http://www.sigmaaldrich.com/catalog/product/fluka/phr1258?lang=en®ion=US
http://www.sigmaaldrich.com/catalog/product/sigma/c6628?lang=en®ion=US
Has anyone ordered from Sigma Aldrich?