Facebook Page/user exist

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Facebook-API

Hello,

My question is:
How can i check an url that my guests can insert on my website?

What i want?
I want to trade with likes (Facebook) but everyone are uploading death links.
How can i check that the page are ok? or what is an easy script?

I know the option graphs.facebook.com but i dont know how i can insert it.

Fill in label on my website:

<tr><td><label for="url">Page URL:</label></td><td width="20"></td><td><input type="text" name="url" id="url" size="40" maxlength="150" value="http://" /></td></tr>

link check on my website (Now):

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if(isset($_POST['add'])){
if(!preg_match("/\bfacebook.com\b/i", $_POST['url'])){$msg = "<div class=\"msg_error\">ERROR: Add your facebook page!</div>";}

How can i add an control to check the pages? i hate death links or private profiles :)