<a href="?address=http://www.website1.com" target="_blank">External Link 1</a>
<a href="?address=http://www.website2.com" target="_blank">External Link 2</a>
<a href="?address=http://www.website3.com" target="_blank">External Link 3</a>
<a href="?address=http://www.website4.com" target="_blank">External Link 4</a>

<?php
$address = (string) $_GET['address'];

$fsock = fsockopen(str_replace('http://','',$address));

…

Tutorial

When linking to an external site you can display a message to the user telling them that the link does not reside on your website.

<a href="?address=http://www.website1.com" target="_blank">External Link 1</a>
<a href="?address=http://www.website2.com" target="_blank">External Link 2</a>
<a href="?address=http://www.website3.com" target="_blank">External Link 3</a>
<a href="?address=http://www.website4.com" target="_blank">External Link 4</a>

These links do not link directly to a website but create a query string address that gives the address of the website.

$address = (string) $_GET['address'];
$fsock = fsockopen(str_replace('http://','',$address));

Now that we have the address we try to open it with the function fsockopen.

if( !$fsock)
   echo 'Address not valid.';

If we cannot open the address then we will alert the user.

else
{
   echo 'Are you sure you want to leave this site?<br />';
   echo 'Destination: <strong>'.$address.'</strong><br />';
   echo '<a href="'.$address.'">Proceed</a>';
}
echo '<a href="javascript:window.close();">Close Window</a>';

Otherwise we display a link to the user and a message telling the user that this link is not within this website.