DocumentRoot /var/www

<Directory /var/www/blah>
    HeaderName /phphtml/HEADER.php
</Directory>

<Directory /var/www/phphtml>
    AddType text/html .php
    <Files "*.php">
       AddHandler application/x-httpd-php .php
    </Files>
</Directory>

# Listings of /blah/ will include the HEADER.php in /phphtml/
# Very limited Options for phphtml is recommended.
# If you normally use AddHandler for your PHP, you should be able to
# remove it from the second <Directory>.