Why are my PHP files showing as plain text? [duplicate]

You should install the PHP 5 library for Apache.

For Debian and Ubuntu:

apt-get install libapache2-mod-php5

And restart the Apache:

service apache2 restart

You'll need to add this to your server configuration:

AddType application/x-httpd-php .php

That is assuming you have installed PHP properly, which may not be the case since it doesn't work where it normally would immediately after installing.

It is entirely possible that you'll also have to add the php .so/.dll file to your Apache configuration using a LoadModule directive (usually in httpd.conf).

You need to configure Apache (the webserver) to process PHP scripts as PHP. Check Apache's configuration. You need to load the module (the path may differ on your system):

LoadModule php5_module "c:/php/php5apache.dll"

And you also need to tell Apache what to process with PHP:

AddType application/x-httpd-php .php

See the documentation for more details.

Yet another reason (not for this case, but maybe it'll save some nerves for someone) is that in PHP 5.5 short open tags <? phpinfo(); ?> are disabled by default.

So the PHP interpreter would process code within short tags as plain text. In previous versions PHP this feature was enable by default. So the new behaviour can be a little bit mysterious.