Linux installation Apache PHP can not parse, how to handle

Linux installation Apache PHP cannot parse
ubuntu12.04 environment with Apt-get installation apache2 and PHP5 LIBAPACHE2-MOD-PHP5 test Apache can access, access to PHP files become download files, download php file shows the results of PHP parsing

------Solution--------------------
Maybe there's a problem with the MIME statement.
You can test it this way:
1, build a a.php content for
Echo __file__;

2, build a b.php content for
Print_r (get_headers (' http://localhost/a.php '));

And then execute the b.php to see what the final result will be.

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