Sure is a long phpBB3 file: functions_content.php
1353 lines.

To give us some hint, can you tell in what part, what lines, you have made changes?
Because I assume a phpBB3 file to be correct out-of-the-box.
You have probably tried to make some MODIFICATIONS.

<?php
/**
*
* @package phpBB3
* @version $Id: functions_content.php 10039 2009-08-21 09:44:55Z bantu $
*/

I was to change this:

$html    = "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";

to

if ($type == MAGIC_URL_EMAIL)
    {
        $html    = "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";    

    }
    else 
    {
        $html    = "$whitespace<!-- $tag --><a$class href=\"$url\" onclick=\"window.open(this.href);return false;\">$text</a><!-- $tag -->$append";
    }

which was somewhere around line 592.

thanks
I will have a look at that part.
Be back

The function you make this changes in
has a missing closing tag,
}
before next function

So, add the last
}
in the end of this code, Line 630 something

/**
* A subroutine of make_clickable used with preg_replace
* It places correct HTML around an url, shortens the displayed text
* and makes sure no entities are inside URLs
*/
function make_clickable_callback($type, $whitespace, $url, $relative_url, $class)
{
//
// ----- //

$url	= htmlspecialchars($url);
$text	= htmlspecialchars($text);
$append	= htmlspecialchars($append);

$html    = "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";  

} // add this in Line 630 or 631

/**
* make_clickable function
*
* Replace magic urls of form http://xxx.xxx., www.xxx. and xxx@xxx.xxx.
* Cuts down displayed size of link if over 50 chars, turns absolute links
* into relative versions when the server/script path matches the link
*/
function make_clickable($text, $server_url = false, $class = 'postlink')

Still I do not understand how that function works.
Because it DOES NOT RETURN anything. At the end
So in my opionion it will be useless!

You are my hero! Thanks so much!

The original function
returns $html;

So, you should add not only the } tag
but this:

	$html	= "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";

return $html;  // the return, result of this function

}     // around Line 630-632