If you see the code, then your Apache isn't set up properly to parse PHP
code. Installing libapache2-mod-php5 should fix that for you. You don't
need both libapache2-mod-php5 and php5-cgi; for now don't even fool with
php5-cgi. And php-auth-http is completely unrelated to your problem.
Installing just libapcahce2-mod-php5 should set up the Apache
configuration for you. If after installing this, you get a 500 error,
you need to look at /var/lib/apache/error.log (or whatever your Apache
error log is called) to see what's happening.