I have heard that PHP is somewhat similar to Java. If you already have some knowledge of programmaing concepts, I think you could pick up basic PHP in 3 weeks. Granted, you won't have much experience with debugging it and such, you could learn the basic functions (strings, loops, etc).
Java is a programming language and PHP is a scripting language. Java has a much higher learning curve than PHP. Anyone with any previous knowledge of logic, etc., could probably pickup PHP in a few days, it really isn't very tough.
PHP *does* have some OO functions built into it, but it's nowhere near complete and just kind of a bad patch. (Thinking of <=v4 here, havenít read anything about 5 just yet.)
If you're thinking of the offline capabilities of it, once again I'll go with the limited functions deal. (Shouldn't take you too long to learn if you can program Java.)
The main differences come with the Object Oriented side of things. You're constantly reminded of the fact you're programming in a OO language with Java. In PHP you have to be really careful how you structure your code, but it is very possible to get a OO feel to it (in particularly with PHP5).
Another thing to bear in mind is that PHP is "loosly" typed. For example: