title: php: only allow form POST from specified domain name
body: <p>I have a page running a php script that subscribes a user to a newsletter, when a form is posted.
  Now, I am worried about getting spammed, cause anyone can post to my page using a form.</p>
  
  <p>Is there any way I can use conditionals in PHP, to only allow incoming submissions from a certain domain?
  If the domain doesn't match, using else to echo an error message.</p>
  
  <p>I am just starting to lean php, so I'd appreciate all the help I can get.</p>
  
  <p>Thank you.</p>