[php] How to make Apache serve index.php instead of index.html?

If I put the following line in a index.html file, to make Apache include the index.php file:

  <?php include("/Live/ls_client/index.php"); ?>    

visiting the index.html page shows me this:

<!--?php include("/Live/ls_client/index.php"); ?-->

Why is that? Why doesn't it actually include the PHP file?

As of today (2015, Aug., 1st), Apache2 in Debian Jessie, you need to edit:

root@host:/etc/apache2/mods-enabled$ vi dir.conf 

And change the order of that line, bringing index.php to the first position:

DirectoryIndex index.php index.html index.cgi index.pl index.xhtml index.htm

PHP will work only on the .php file extension.

If you are on Apache you can also set, in your httpd.conf file, the extensions for PHP. You'll have to find the line:

AddType application/x-httpd-php .php .html
                                     ^^^^^

and add how many extensions, that should be read with the PHP interpreter, as you want.