PHP if-Funktion für Variable aus URL

<?php echo $HTTP_GET_VARS[img]; ?>

<textarea (...)><img src="images/<?php echo $HTTP_GET_VARS[img]; ?>"></textarea>

<textarea (...)><img src="images/"></textarea>

<?php if($img = '')
  {
    echo '';
   }
   else
   {
    echo '<img src="images/';
    echo $HTTP_GET_VARS[img];
    echo '">';
   }
?>

if(isset($_GET["img"]))
{
}

<?php if(isset($_GET["img"])) 
  { 
    echo '<img src="images/'; 
    echo $HTTP_GET_VARS[img]; 
    echo '">'; 
   } 
   else 
   { 
    echo '';
   } 
?>

<?php
include('datenbank.php');
$db2 = mysql_query("SELECT * FROM `posts` WHERE `kategorie` = '$_GET[kategorie];'");
while($db3 = mysql_fetch_array($db2)){
echo '
<div class="box">
<h1>'.$db3[titel]. '</h1>
<p>'.$db3[inhalt]. '</p>
</div>
';
}
?>

<img src="images/<?php echo $HTTP_GET_VARS[img]; ?>">

$db2 = mysql_query("SELECT * FROM `posts` WHERE `kategorie` = '$_GET[kategorie];'");

<?php
include('datenbank.php');
$db2 = mysql_query("SELECT DISTINCT kategorie FROM posts ORDER BY kategorie ASC");
$count = mysql_query("SELECT DISTINCT COUNT(*) FROM `posts` WHERE `kategorie` = 'wallpaper'");
while($db3 = mysql_fetch_array($db2) OR $zahl = mysql_fetch_array($count)){
echo '
<a href="kategorie.php?kategorie='.$db3[kategorie].'"><div class="box-klein">
	<h1>'.$db3[kategorie].'</h1>
	<h2>'.$zahl.' Post(s)</h2>
</div></a>
';
}
?>

$sql = "SELECT DISTINCT
	`kategorie`,
	(SELECT DISTINCT COUNT(*) FROM `posts` WHERE `kategorie` = '" . mysql_real_escape_string($_GET['kategorie']) . "') AS `anzahl`
FROM `posts`
WHERE `kategorie` = '" . mysql_real_escape_string($_GET['kategorie']) . "'
ORDER BY kategorie ASC"

echo '<a href="kategorie.php?kategorie='.$db3['kategorie'].'"><div class="box-klein">
<h1>'.$db3['kategorie'].'</h1>
<h2>'.$db3['anzahl'].' Post(s)</h2>
</div></a>';

WHERE ... = '$wallpaper'

" . mysql_real_escape_string($_GET['kategorie']) . "