Waarom werkt dit niet?

Waarom werkt dit niet?
http://www.lerisranantongo.com/v1/woorden?act=show&t=1

Code:

[COLOR= #0000BB]<?php[/COLOR]
[URL="http://www.php.net/require_once"]require_once[/URL] [COLOR= #DD0000]inc/header.php[/COLOR][COLOR= #007700];[/COLOR]
 [URL="http://www.php.net/if"]if[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$_GET[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]act[/COLOR][COLOR= #007700]] ==[/COLOR] [COLOR= #DD0000]show[/COLOR][COLOR= #007700])
    {[/COLOR]
   [URL="http://www.php.net/if"]if[/URL][COLOR= #007700](([/COLOR][URL="http://www.php.net/isset"]isset[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$_GET[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]t[/COLOR][COLOR= #007700]])) || ([/COLOR][URL="http://www.php.net/ctype_digit"]ctype_digit[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$_GET[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]t[/COLOR][COLOR= #007700]])))
        {[/COLOR]
            [COLOR= #0000BB]$topic_info_query[/COLOR] [COLOR= #007700]=[/COLOR] [URL="http://www.php.net/mysql_query"]mysql_query[/URL][COLOR= #007700]([/COLOR][COLOR= #DD0000]"SELECT * FROM `woordenlijst` WHERE `id` = "[/COLOR] [COLOR= #007700].[/COLOR] 

[URL="http://www.php.net/mysql_real_escape_string"]mysql_real_escape_string[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$_GET[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]t[/COLOR][COLOR= #007700]]) .[/COLOR] [COLOR= #DD0000]""[/COLOR][COLOR= #007700])[/COLOR] or [URL="http://www.php.net/trigger_error"]trigger_error[/URL][COLOR= #007700]([/COLOR][URL="http://www.php.net/mysql_error"]mysql_error[/URL][COLOR= #007700]());[/COLOR]
            [URL="http://www.php.net/if"]if[/URL][COLOR= #007700]([/COLOR][URL="http://www.php.net/mysql_num_rows"]mysql_num_rows[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$topic_info_query[/COLOR][COLOR= #007700]))
            {[/COLOR]
                [COLOR= #0000BB]$topic_info[/COLOR] [COLOR= #007700]=[/COLOR] [URL="http://www.php.net/mysql_fetch_assoc"]mysql_fetch_assoc[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$topic_info_query[/COLOR][COLOR= #007700]);
                {[/COLOR]
                    [COLOR= #0000BB]$cat_sql[/COLOR] [COLOR= #007700]=[/COLOR] [URL="http://www.php.net/mysql_query"]mysql_query[/URL][COLOR= #007700]([/COLOR][COLOR= #DD0000]"SELECT * FROM `woorden` WHERE `id` = "[/COLOR] [COLOR= #007700].[/COLOR] 

[URL="http://www.php.net/mysql_real_escape_string"]mysql_real_escape_string[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$topic_info[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]cat_id[/COLOR][COLOR= #007700]]) .[/COLOR] [COLOR= #DD0000]""[/COLOR][COLOR= #007700])[/COLOR] or [URL="http://www.php.net/trigger_error"]trigger_error[/URL][COLOR= #007700]([/COLOR][URL="http://www.php.net/mysql_error"]mysql_error[/URL][COLOR= #007700]());[/COLOR]
                    [URL="http://www.php.net/if"]if[/URL][COLOR= #007700]([/COLOR][URL="http://www.php.net/mysql_num_rows"]mysql_num_rows[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$cat_sql[/COLOR][COLOR= #007700]))
                    {[/COLOR]
                        [COLOR= #0000BB]$cat[/COLOR] [COLOR= #007700]=[/COLOR] [URL="http://www.php.net/mysql_fetch_assoc"]mysql_fetch_assoc[/URL][COLOR= #007700]([/COLOR][COLOR= #0000BB]$cat_sql[/COLOR][COLOR= #007700]);
                        {[/COLOR]

[URL="http://www.php.net/echo"]echo[/URL] [COLOR= #DD0000]<h3>Leer de woorden</h3>[/COLOR][COLOR= #007700];[/COLOR]
            
            [URL="http://www.php.net/echo"]echo[/URL] [COLOR= #DD0000]<textarea onfocus="blur()">[/COLOR][COLOR= #007700].[/COLOR]htmlspecialchars[COLOR= #007700]([/COLOR][COLOR= #0000BB]$cat[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]woord[/COLOR][COLOR= #007700]]).[/COLOR][COLOR= #DD0000] - 

[/COLOR][COLOR= #007700].[/COLOR]htmlspecialchars[COLOR= #007700]([/COLOR][COLOR= #0000BB]$cat[/COLOR][COLOR= #007700][[/COLOR][COLOR= #DD0000]antwoord[/COLOR][COLOR= #007700]]).[/COLOR][COLOR= #DD0000]</textarea><br />[/COLOR][COLOR= #007700];
                }

            }
        }
        }
    }
}[/COLOR]
[URL="http://www.php.net/require_once"]require_once[/URL] [COLOR= #DD0000]inc/footer.php[/COLOR][COLOR= #007700];[/COLOR]
?>

cat_id bestaat niet ?