I would venture a guess that each of your 50 pages are almost identical and only differ in the content that is on each page?
If so, this is an ideal candidate for having one page of code using php and simply displaying the different content by having php populate it from a database depending on what navigation link is selected.
If you are going to go through the trouble of modifying each page to change headers, footers, and navigation to use includes, why not put that same amount of effort and time (or it could actually be less) into reducing it to one page with the different content for each page being stored in a database.
If this is something like repetitive product or any other repetitive information display, you will end up saving yourself a lot of time long term by doing it this way.
If you are learning PHP, developing PHP code, or debugging PHP code, do yourself a favor and check your web server log for errors and/or turn on full PHP error reporting in php.ini or in a .htaccess file to get PHP to help you.