By adding AddType x-mapp-php5 .php to your .htacess file you will tell 1and1 to use php5 instead of the default php4. It should look something like this
AddType x-mapp-php5 .php
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
