html - Set HTTP header to UTF-8 using PHP

I have several PHP pages echoing out various things into HTML pages with the following code.

However, when I validate using the W3C validator it comes up with:

The character encoding specified in the HTTP header (iso-8859-1) is different from the value in the element (utf-8).

I am quite new to PHP, and I was wondering if I could and should change the header for the PHP files to match the HTML files.