Objective-C is a compiled language, strong typed, then is different to Php in many ways.

Since 2.0 version, Obj-C has its own “garbage-collector” for memory management. I don’t try to develop since 2.0 release so I can’t tell about this auto memory management. I learn Objc-C by myself last summer with Apple Documentation (very useful and complete) , and by buying an Oreill’y book (Cocoa Programming for 10.2). Problem is that there is not an 10.5 book edition at the moment, and you will have to wait for a long time i think.

Here a basic pdf found on ADC, http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/ObjC.pdf

You can find more by registering for free on ADC website, to download samples or XCode latest version. http://developer.apple.com/

What type of app do you want to do with Objective-C ? Cocoa framework is really interesting, you should take a look at it !

You should think about what type of software you want to code and who your audience is. PHP is better for web apps than Obj-C, but Obj-C is better for other things. If you’re just looking for a new language to learn, C is an important one. :)