Solution using Google Page Speed - tested & working.
//SOLUTION 1
<?php
$link = "http://example.com";
$googlePagespeedData = file_get_contents("https://www.googleapis.com/pagespeedonline/v2/runPagespeed?url=$link&screenshot=true");
$googlePagespeedData = json_decode($googlePagespeedData, true);
$screenshot = $googlePagespeedData['screenshot']['data'];
$screenshot = str_replace(array('_','-'),array('/','+'),$screenshot);
$show_link = "<a href='$link'><img src=\"data:image/jpeg;base64,".$screenshot."\" /></a>";
echo $show_link;
//SOLUTION 2
$name = 'test';
$googlePagespeedData = file_get_contents("https://www.googleapis.com/pagespeedonline/v2/runPagespeed?url=$link&screenshot=true");
$googlePagespeedData = json_decode($googlePagespeedData, true);
$screenshot = base64_decode($googlePagespeedData['screenshot']['data']);
$data = str_replace('_','/',$googlePagespeedData['screenshot']['data']);
$data = str_replace('-','+',$data);
$decoded = base64_decode($data);
file_put_contents('myfolder/'.$name.'.jpg',$decoded);
$file_name = "$name.jpg";
/*
-- IMPORTANT INFORMATION -- READ BELOW --
Choose how to proceed!
1. Use the above to display screenshots of links = longer processing time for multiple links.
2. Save image to a file, reference the saved image = more disk space needed if multiple links.
Note the trade off between processing time and disk space, if you're on a shared hosting platform with a small disk space limit and envisage or already have a lot of users (forums beware) you may want to consider a bigger hosting plan or even a dedicated server.
*/
?>