Below are two samples of what you can use in a .htaccess to force SSL on your PHP4 or PHP5 site:

Force SSL on entire site
RewriteEngine On
RewriteBase /
RewriteCond %{ENV:HTTPS} !on [NC]
RewriteRule ^(.*)$ https://%{SERVER_NAME}/$1 [R,L]

Force SSL on a specific directory
RewriteEngine On
RewriteBase /
RewriteCond %{ENV:HTTPS} !on [NC]
RewriteRule ^(.*)$ https://%{SERVER_NAME}/DIRNAMEHERE [R,L]