Cocoa Dev Central has a tutorial for Cocoa (Mac) which assumes only basic PHP knowledge. Check it out:

First, I assume that you have a basic, non-expert understanding of PHP. I think this is a safe assumption because most people seem to play around with a scripting language before they approach Cocoa. PHP is widely used, and the syntax is very similar to C. Second, I assume that your end goal is to write Cocoa programs in Objective-C.