CSS(cascading style sheets) is one other field to know if you are worrying about user interface in you application. If you don't have basic knowledge in above fields (HTML and CSS) and like to learn them while learning php its also good.
Knowledge from other programming language like c,c++,c#,java will speedup the learning process but it is optional. You will learn that php also have same functional things like arrays, loops..etc. If you know when to use arrays, loops.. it is a advantage for learning php algorithms.
Having a logical mind is important to learn and use php successfully in your applications. I would say it may be little bit hard to learn php as a novice without logical thinking skills. But if you are a hard worker practicing some more excises and reading tutorials again and again will do the job. Also Remember that you should do more excises than reading tutorials with learning from your mistakes. Php has a good error reporting system which will ease the job. Stackoverflow is a good place to get help regarding your php problems. Stackoverflow has great community and your problem will be answered within minutes.
