Is it important to learn Java in order to become software developer?

Being a software developer is independent of which programming languages you know.  You can build your development skills using any language and it will transfer readily to other languages.  The skills that you need to learn are how to take business requirements and turn them into software.  Source code management, versioning, branching, packaging releases, patches, unit testing, automation, and code deployment are all applicable no matter the language.  I would start with what you know PHP, javascript, CSS to learn general development practices. As you build larger systems, performance tuning and scalability will be required and you can explore other technologies.  For example, you may need caching to improve performance, or a distributed database.  Once you have developed these general skills, learning new languages and frameworks will be easy because the principles are the same in any language.  I would also focus more on the types of applications you can build - mobile apps, web, responsive, web services, data processing - instead of the languages you know.  This is what companies will be interested in and will make you stand out among the large crowd.

Java sets you up to develop web apps whilst it is only one step from learning Android and thus mobile apps. That gets my vote.

My suggestion is to focus a little less on the tools, and a little more on the theory and the craft. You're not a designer just by knowing Photoshop, and you're not necessarily a software developer even if you know a few programming languages (even Java). Practice your craft. Get help from a tutor. Get a degree if you can. Read a few books. Practice some more. Publish something. HTML, CSS, JavaScript and php are excellent tools, capable of building awesome things. If and when you truly know these tools and are able to build something useful with them, you are a Software Developer. That said, we all benefit from picking up new tools. Go ahead and try Java. Or maybe Python. - Python's fun.

It can help but you don't need to.  Big and medium sized companies tend to use Java and C##.  So you might need that to get into one of those, but it might be hard for you to get into one of those anyway. This is just my personal opinion, but I think PHP isn't that good and you should consider learning Ruby, Python, or Node.js.

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