One can use vim
for that:
find -type f \( -name '*.css' -o -name '*.html' -o -name '*.js' -o -name '*.php' \) -execdir vim -c retab -c wq {} \;
As Carpetsmoker stated, it will retab according to your vim
settings. And modelines in the files, if any. Also, it will replace tabs not only at the beginning of the lines. Which is not what you generally want. E.g., you might have literals, containing tabs.