How to recognize Facebook User-Agent

Question

When sharing one of my pages on FB, I want to display something different. Problem is, I prefer not to use the og: elements, but to recognize FB user-agent.

What is

Answer 1

Another generic approach in PHP

$agent = $_SERVER['HTTP_USER_AGENT'];
$agent = trim($agent);
$agent = strtolower($agent);
if (
strpos($agent,'facebookexternalhit/1.1')===0
|| strpos($agent,'facebookexternalhit/1.0')===0
){
    //probably facebook
}else{
    //probably not facebook
}