Inserting PHP code into an HTML document.

Why is it necessary to use a PHP script?  JavaScript solutions are everywhere.

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If you do change all pages to a .php extension you may want to consider the impact and set up a redirect, this is assuming you are currently serving your pages with a .html extension (its more the norm these to serve pages without an extension). In this case you'll can use mod_rewrite to redirect all old .html files to their .php equivalent with something like this in htacess file:

RewriteEngine On
RewriteRule ^(.*)\.html$ $1.php [L, R=301]

This keeps everything tidy and it guards against you losing any benefit of established pages that are generating traffic.

1. Does your webhosting account support PHP?
2. If yes, is your webhosting account (the server) configured to parse HTML documents and look for and process PHP code? Ask your webhost.
3. If not, you'll have to rename the page with a .php extension and update any links which point to it.
4. No need to convert the entire site although for consistency it's often good practice, even for pages with no PHP code in them.