Hy, I know Html, css quite well. But I want to be WordPress expert. So I want to know how much php skill should I have to be php Expert.
Thanks
I think you meant “be wp Expert”.
However, first of all, why would you base your entire career on a single CMS?!
It’s not even a framework. I agree that you can make money with it but, it’s limited.
To create a theme, and only a theme, you just need to know the PHP syntax basic, to avoid syntax errors, that’s it as PHP knowledge.
The rest it’s about HTML and structure.
To create a plugin, you need to know PHP. Here it’s not about “how much” because this is not a limited thing.
It’s not like “you can learn everything but you don’t know if it’s worth it”. You cannot learn everything about programming.
Just learn as much as you can, make experiments, create projects.
Does it worth it?
Yes, if you like it.
No, if you’re doing it just for money.
Yes, it helps to learn PHP, especially if you want to customize or develop themes and/or plugins.
Sitepoint has a number of PHP books available: http://products.sitepoint.com/?tag=&filters[tag][]=php&filters[difficulty]=&simpleform_submit_marker=showme
And also this book, which introduces you to wordpress development: http://www.sitepoint.com/books/wpant1/
Well…yes and no. Granted, it’s not generally a good idea to put all your eggs in once basket. However, you have to start the learning process somewhere.
Plus, wordpress is extremely popular. There are a handful of web development shops in my areas that primarily focus just on developing with wordpress.
don’t need to know a lot of actual php. just be really knowledgeable in wordpress loops hooks and filters. it’s all in the wp codex with examples
The hooks and filters are based on PHP, so yes, you still will need to learn PHP in order to use them effectively.
The Wordpress Codex only offers very basic examples. You’re on your own if you want to customize things.
this was just based on my personal experience. i’m pretty familiar with custom loops queries and functions. i’m not sure if the knowledge would carry over in a non-wp php project since the only php i do is wp. If it does though then that’s great news! 
WordPress is not so simple as it looks like. Just my opinion: starting with “looking over a CMS” is not a good start.
First, I bet he’ll get bored as hell and, assuming he’ll be a fast learner, he’ll skip lots of basic things. To learn programming you have to start with the beginning, to know what an SQL injection is, to know how to create your own class.
Second, after such a start, if you want to hire into a real company (to work on something real big) you will get all from zero, it will be very difficult to adapt to new things, after your entire process of learning was based on a single simple structure.
Hooks and Filters are wordpress-specific.
Function usage and syntax are the things that would carry over.
That’s somewhat subjective. It depends upon what needs to be accomplished.
True, it’s best to learn PHP as a stand-alone language first before diving into wordpress.