I have migrated 2 or 3 sites to silverstripe that were mainly php and html.
My approach was like that:
- Install silverstripe on the main directory. If your site has .htaccess already changed you have to move carefully.
- Create the infrastructure on silverstripe in case you have some pages and menus
- Copy the content page by page (by copying the content the content will keep the reference to existing images as they are
- Fix things like tables as they can have problems in fitting in the columns
- If you have installed some program in a sub folder when selecting the folder instead of the program it will show the not found page. You need to write the full page adress like www.domainname.com/forum/index.php. I have created pages to state that if you are arriving here you should look at the page with the full address
- Change the layout to better fit your content.
- Add images to Silverstripe and make the include of the images in the pages instead of the link to the old images
- Replace the links in the middle of the text to silverstripe pages instead of the existing old pages
After migrating the content it is easier to keep it updated and to divide the pages in more sub-pages.
Try to use the redirect options from the account control panel to redirect the users to the new pages.
Remember that you should have users that are pointing still to the old pages.
Hope that this can help someone.