What is the use of header() function in php?

PHP's header function allows you to change the headers on your returned content so that (examples)

• The browser receives not HTML but plain text, a .jpg image or a file to save locally
example: header ('Content-type: Application/Octet-stream');

• You can change a "404" missing reponse code into a "200" - good.
example: header("HTTP/1.1 200 OK");

• You can send out cache instructions
example: header("cache-control: no-store");

• you can sent out a save file name if you're saving the file.
example: header ('Content-Disposition: attachment; filename="hello.txt"');