STR_REPLACE

Kode 

<form method="post" action="side.php">
www.<input type="text" name="link">
</form>

Kode 

<?php
$link = $_POST['link'];


$rawstring = "Dit link er lige her: LINK";


$str = str_replace("LINK", "<a href=\\"http://www.$link\\">$link</a>", $rawstring);

echo "". $str . "<br />";
?>

Kode 

<?php
	$text = 'asdasd http://www.dr.dk asdasd http://www.doo.dk';
	$text = preg_replace('^(([\\w]+:)?//)?(([\\d\\w]|%[a-fA-f\\d]{2,2})+(:([\\d\\w]|%[a-fA-f\\d]{2,2})+)?@)?([\\d\\w][-\\d\\w]{0,253}[\\d\\w]\\.)+[\\w]{2,4}(:[\\d]+)?(/([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})*)*(\\?(&?([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})=?)*)?(#([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})*)?^','<a href="$0">$0</a>',$text);
	print $text;
?>

Kode 

<?php
	$text = 'asdasd http://www.dr.dk asdasd http://www.doo.dk';
	$text = preg_replace('^(([\\w]+:)?//)?(([\\d\\w]|%[a-fA-f\\d]{2,2})+(:([\\d\\w]|%[a-fA-f\\d]{2,2})+)?@)?([\\d\\w][-\\d\\w]{0,253}[\\d\\w]\\.)+[\\w]{2,4}(:[\\d]+)?(/([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})*)*(\\?(&?([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})=?)*)?(#([-+_~.\\d\\w]|%[a-fA-f\\d]{2,2})*)?^','<a href="$0">$0</a>',$text);
	print $text;
?>