htaccess rewrite subdomains to 1 file - .htaccess

Question

I am helping someone with their site and they want a template page (example.com/template) shown on each of their subdomains.
So when a user navigates to a.example.com or b.example.com, etc.they see example.com/template.php but the url stays a.example.comor b.example.com, etc.
I know there is a way to do this I just can't figure it out.
Basically I want to be able to change all of the pages at once by making a change to 1 file.
I think I am close with this code but I keep get 500 errors
RewriteCond %{HTTP_HOST} ^$.example.com$ [NC]
RewriteRule ^$ $1.example.com/template.php
PS: these are actual domains, not virtual
This is what I currently have
RewriteEngine On
RewriteCond %{HTTP_HOST} ^.+\.dfwfamilylifenews\.com [NC]
RewriteRule ^$ /template.php [L]
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Answer 1

Firstly, make sure the rewrite module is loaded in Apache. See this answer for more info.
Second, you need to change your rules to:
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^.+\.example\.com [NC]
RewriteRule ^$ /template.php [L]