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PHP tags inside an HTML document are no different then PHP tags inside a PHP document. The real issue is what interpreter is parsing your HTML files. You need to make sure Apache uses the PHP parser on all files that have the .HTML extension.
AddType application/x-httpd-php .html
AddType application/x-httpd-php3 .html
but this will make the php engine to parse all html files, so make sure that's what you want.