tar -cf my_archive `find ./ | grep '.php\|.html'`
Use "find" and "grep" to get all path of .php and .html files in all directory and its sub-directories. Then pass those path information to tar to compress.
Please be careful with those symbol ` and '. Note also that this will hit the limit of how many characters your shell will allow on the command line, unlike some of the other answers.