When someone says something like that, you need them to justify what they say. PHP is scalable and versatile. Java has its own strengths and weaknesses, like any other programming language.
Since you're new to PHP, you probably won't reuse most of the code that you use to create the source site. I know what you are planning, but the likelihood of what you do is very slim. Go with what you know.
PHP is used by many sites. Google, Yahoo, Facebook, etc. Etc. Everyone uses some php for all of their sites.
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