How to find and replace in specific file type everywhere in a directory

I want to find a keyword in only php files and replace the same with new keyword, How can I do it in linux?

You can use find command.

-type f :look into files only
-name '*.php' :for file extension php only
-exec :executing grep

find ./ -type f -name '*.php' -exec sed -i -e 's/OldKeyword/NewKeyword/g' {} \;

If you want to just find a keyword in only specific file type let's say php files. You can use find command.

-type f :look into files only
-name '*.php' :for file extension php only
-exec :executing grep

find ./ -type f -name '*.php' -exec grep -H --line-number --color "my keyword" {} \;