By default phpmyadmin (on debian, right?) installs a private .htaccess file which enables php parsing for the phpmyadmin directory. Here is the default .htaccess file phpmyadmin on debian:
Code:
DirectoryIndex index.php
Options +FollowSymLinks
<IfModule mod_mime.c>
<IfModule mod_php4.c>
AddType application/x-httpd-php .php
php_flag magic_quotes_gpc Off
php_flag track_vars On
php_flag register_globals Off
php_value include_path .
</IfModule>
<IfModule mod_php5.c>
AddType application/x-httpd-php .php
php_flag magic_quotes_gpc Off
php_flag track_vars On
php_flag register_globals Off
php_value include_path .
</IfModule>
<IfModule mod_php5_filter.c>
AddType application/x-httpd-php .php
php_flag magic_quotes_gpc Off
php_flag track_vars On
php_flag register_globals Off
php_value include_path .
</IfModule>
<IfModule mod_php5_hooks.c>
AddType application/x-httpd-php .php
php_flag magic_quotes_gpc Off
php_flag track_vars On
php_flag register_globals Off
php_value include_path .
</IfModule>
<IfModule !mod_php4.c>
<IfModule !mod_php5.c>
<IfModule !mod_php5_filter.c>
<IfModule !mod_php5_hooks.c>
<IfModule !mod_fastcgi.c>
<IfModule !mod_fcgid.c>
<IfModule mod_actions.c>
<IfModule mod_cgi.c>
AddType application/x-httpd-php .php
Action application/x-httpd-php /cgi-bin/php
</IfModule>
</IfModule>
</IfModule>
</IfModule>
</IfModule>
</IfModule>
</IfModule>
</IfModule>
</IfModule>
From the behavior you are seeing, it seems like you have php installed and running (since myadmin is parsing properly), but the 'AddType' directive is not getting inserted into the other part of your site that isn't parsing php directly.
Were you using apache1 or apache2? It has been a while since I used debian apache1 so....
On apache2, check out /etc/apache2/mods-enabled and ensure there is a soft link for php4.conf and php4.load. If so I'm all outta suggestions except for perhaps to check out your /etc/apache2/apache2.conf or /etc/apache/httpd.conf and manually insert the AddType directives from above.