when we write a php file and save it in apache web folder it works well....
If it is outside the apache it doesnt works ...why?

Keep in mind that it is Apache that is loading the php module(s) that will interpret and present content to your browser. A browser alone most likely wont know what to do with a php file.

It's exatly what happens if you try to open a bash script in your favourite editor: the script won't be executed, you'll just be able to edit it.

1)Some php files can be opened with a browser by renaming : mv 'file.php' 'file.html'
2)Opera is known to open some php files.

I think what the OP is referring to is that directly opening a .php file in a web browser gives you the PHP code, not the page as it looks when you request it from the server. Renaming or using Opera won't help with that.

Long story short, when you request a .php page through a web server, you're not seeing the .php file - you're seeing the output of an executed PHP program. When you request the file from Apache, it sees that the file is a PHP script, runs it through the PHP interpreter, and sends you the output. When you open the file in your browser, or request it from a server that doesn't have PHP installed, it's just treated as yet another text file. So it never actually gets executed and hence does not "work".

Not sure what "it works" means or in what program or context it "works". But if not working means that visiting that page doesn't produce the expected output, perhaps www-data doesn't have execute or other necessary privileges on files "outside the apache."