Pages won't validate as XHTML because of Char Encoding

In order to make my pages valid XHTML 1.0, I am required to specify a character encoding. However, in order to be able to easily update this encoding in the future, I do not want to use the meta tags.

Usually, this can be done server side by putting the following into the HTACCESS file:

AddType 'text/html; charset=ISO-8859-1' shtml

However, my pages are PHP, not SHTML. Therefore, when I type PHP instead of SHTML in that line, it disables my PHP pages from working :-(

Does anyone know a way to set this character encoding another way? I don't want to use meta tags, and htaccess doesn't seem to work PHP wise...