Assume you have to following PHP file which has an ISO-8859-1 file encoding
echo "This is a string which contains an é";
and the http charset of the page sent to the client browser is set to UTF-8
The browser will propable show a a question mark inside of a black diamond for characters that cannot be displayed properly. To solve this issue be sure to set the file encoding / charset for both files to UTF-8 or ISO-8859-1. In this case I should go for UTF-8.