Convert tabs to 4 spaces and trim any trailing whitespace

convert-tabs-and-trim-trailing-whitespace.sh

# This requires GNU sed (or any sed which supports -i) and expand

find . -type f -printf "sed -i 's/[ \t]*$//' %p && expand -t 4 %p > %p.tmp && mv %p.tmp %p\n" | sh

# It's better to be more specific about which files you want to run the replacement in, e.g.
find . -type f -name *.php -printf "sed -i 's/[ \t]*$//' %p && expand -t 4 %p > %p.tmp && mv %p.tmp %p\n" | sh

# You can use find's -regex option to match multiple extensions (you need to double-escape backslashes in bash):
find . -type f -regex .+\\\.\\\(php\\\|css\\\|js\\\)\$ -printf "sed -i 's/[ \t]*$//' %p && expand -t 4 %p > %p.tmp && mv %p.tmp %p\n" | sh