Rename all *.html files in one folder
rename
#rename .html .php *.html
Do the same operation recursively in a directory tree
find . -name "*.html" -exec rename .html .php {} \;
Those double quotes around *.info are important
If you want to be expert go to following link and don't forget to read all the comments :)
http://www.thegeekstuff.com/2009/06/how-to-rename-files-in-group/