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During development (debugging) the site works well but when I release from the debugging and test into production.. the result from PHP file was not displayed.
My initial reaction is to put "session_start();" before you send any headers but that usually throws up an error. Have you tried the page without including your classes?. It won't work but it might show whether it is the include that is causing you problems.
The other thing you can do is just echo "hello world" at the very bottom of the page. That would show if the page is encountering an error and exiting or if it is the script its self causing a problem. It may just be because your SQL query is returning nothing. Try this just to check.
When a page is delivered that you expect to contain HTML generated by a PHP script and it does not, it is usually because the PHP contains a syntax error. When that is true, PHP just won't run the script, so it does not generate any HTML. I hate that "feature" of PHP. The question is why it runs okay in the development environment, but not when you go to production. I would still suspect a syntax error, though. Is it at all possible that you made some minor change in the PHP code between the last time you tested it in development and when you put it into production? All it takes is one missing ; or something.
If what you showed us was the complete PHP code, though, as mentioned by Steve, you do need to have a session_start(); statement before you assign or try to read a session variable. But that would not cause the problem you described.