I'm wondering if multiple entries on htaccess will work for 301 redirects.
The problem I see is that old site files have 'html' extension and are named named differently so a simple global redirect won't work, it has to be one rule per file name.
This sites receives 1000 visits daily so need to be careful not to penalize search engine.
RewriteEngine On
RewriteBase /
RewriteRule ^file1\.html$ http://www.domain.com/file1.php [R=301,NC,L]
RewriteRule ^file2\.html$ http://www.domain.com/file2.php [R=301,NC,L]
RewriteRule ^file3\.html$ http://www.domain.com/file3.php [R=301,NC,L]
RewriteRule ^file4\.html$ http://www.domain.com/file4.php [R=301,NC,L]
A php header rewrite will not work as the old files are html type.