HELP! posting url's in forms with PHP

[ndogg]

I am trying to make something similar to http://www.yellow5.com/magic/magicurl.php . I have a page called index.html with the following:

<table> 
<tr> 
<td> 
<FORM METHOD=GET ACTION="submit.php"> 
Your URL: 
<INPUT NAME="url" TYPE="TEXT" value="http://" size="50" maxlength="100"> 
</td> 
<td align="left" valign="top"> 
<input type="submit" name="submit" value="SPAM!"> 
</td> 
</tr> 
</table>

and then a file called submit.php...

<? 

if ($url != "") { 
$url = htmlspecialchars("$url", ENT_QUOTES); 
$filename = 'list.txt'; 
$fp = fopen($filename, "a"); 
$string = $url . "\n"; 
$write = fputs($fp, $url . "<br>"); 
fclose($fp); 

echo ($url . " written to the file"); 

} 

?> 
</b> 
<hr> 
<? 
include("list.txt"); 
?>

Well in my case the value "$url" needs to have HTML in it to make it in "a href" (link) but when I have "htmlspecialchars" assigned to the variable it only shows up as HTML code, sure I could get it to work without the 'htmlspecialchars' but then it allows people to change text size, parse PHP, etc...

So this one question needs to be answered: How do you only let links be parsed but no other HTML.

[bretticus]

Instead of simply using htmlspecialcharacters, I would validate the URL via regular expression test.

You'll notice that this forum, for instance, won't allow you to post HTML. They strip it I believe, instead they use psuedo-code. You know, the [ then PHP then a ], that allows you to tighten down what the user can input into your page.

[ndogg]

do you know how I code do that? Code you provide some code?