Update sympl-phpmyadmin.conf to match stretch.

phpmyadmin/apache2/conf-available/sympl-phpmyadmin.conf

 # Relax PHP Restrictions
 <Directory /usr/share/phpmyadmin >
-  php_admin_value open_basedir /usr/share/phpmyadmin:/usr/share/php:/etc/phpmyadmin
+  php_admin_value open_basedir /usr/share/phpmyadmin:/usr/share/php:/etc/phpmyadmin:/var/lib/phpmyadmin/tmp
 </Directory>
 
-<IfModule rewrite_module>
-  # Force phpMyAdmin to use HTTPS.
-  RewriteCond "%{HTTPS}" "off"
-  RewriteCond "%{HTTP_HOST}" =""
-  RewriteRule "^/?(phpmyadmin.*)$" "https://<%= hostname %>/$1" [R=301,L]
-  RewriteCond "%{HTTPS}" "off"
-  RewriteRule "^/?(phpmyadmin.*)$" "https://%{HTTP_HOST}/$1" [R=301,L]
-</IfModule rewrite_module>
+<LocationMatch "^/phpmyadmin/">
+  # Enforce HTTPS for phpMyAdmin URLs
+  <IfModule rewrite_module>
+    # Enable rewrites
+    RewriteEngine On
+
+    # If HTTPS is not being used...
+    RewriteCond %{HTTPS} off
+    # ... and there is no HTTP_HOST being sent ...
+    RewriteCond "%{HTTP_HOST}" =""
+    # ... send the traffic to HTTPS on the SERVER_NAME
+    RewriteRule "(.*)$" https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]
+
+    # If HTTPS is not being used...
+    RewriteCond %{HTTPS} off
+    # ... send the traffic to HTTPS on the HTTP_HOST
+    RewriteRule "(.*)$" https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
+  </IfModule>
+</LocationMatch>