One method is to simply have your webserver treat .html files as PHP files.
That way your links can stay intact, and progressively replacing static .html pages with actual php-enabled pages can be done in an transparent method to users.
So There are only files that contain code blocks, which will get interpreted/executed when the containing file is passed through PHP.
But more server are disable this method for security problem.
Another way is to convert html code to php format not so code block inside html script
You can download this sample file exe to convert your file html to php
Open create_php.exe and select 'select file'
save as 'your.php file' in sample case cadoptimize.html convert to cadoptimize.php
This is the html text
This is the html text convert to php
The *.rar file contains one file create_php.exe
The demo of this software is provided FREE by cadoptimize.com