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

Firstly you should not use in_array as you will need to have the full user agent and not just a subset, thus will quickly break with changes (i.e. version 1.2 from facebook will not work if you follow the current preferred answer). It is also slower to iterate through an array rather than use a regex pattern.

As no doubt you will want to look for more bot's later so I've given the example below with 2 bot names split in a pattern with the pipe | symbol. the /i at the end makes it case insensitive.

Also you should not use $_SERVER['HTTP_USER_AGENT']; but you should filter it first incase someone has been a little nasty things exist in there.

$pattern = '/(FacebookExternalHit|GoogleBot)/i';
$agent = filter_input(INPUT_SERVER, 'HTTP_USER_AGENT', FILTER_SANITIZE_ENCODED);
    if(preg_match($pattern,$agent)){
      echo "found one of the patters"; 
   }

A bit safer and faster code.