search and replace string on multiple files from unix terminal
Try this:
find . -name '*.html' -exec sed -i 's/\.html/\.php/g' "{}" \;
It will find all files in the current and subdirectories that end in .html, and run sed on each of them to replace .html with .php anywhere it appears within them.
See http://content.hccfl.edu/pollock/unix/findcmd.htm for more details.