Image captcha

$code         = str_replace(" ", "",$code); 
if($_POST['captcha'] != "" && $_POST['captcha'] != $_SESSION['captcha']){
$error = 3;
}




function rand_str($length = 32, $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890')
{
    // Length of character list
    $chars_length = (strlen($chars) - 1);


    // Start our string
    $string = $chars{rand(0, $chars_length)};
   
    // Generate random string
    for ($i = 1; $i < $length; $i = strlen($string))
    {
        // Grab a random character from our list
        $r = $chars{rand(0, $chars_length)};
       
        // Make sure the same two characters don't appear next to each other
        if ($r != $string{$i - 1}) $string .=  $r;
    }
   
    // Return the string
    return $string;
}
$aantal_codes = 10;
$codes = array();
for($x = 0;$x < $aantal_codes;$x++)
{
    $codes[$x] = md5(rand_str(60));        
}
$code_id = rand(0, 4);
$juiste_code = $codes[ $code_id ];
$_SESSION['captcha'] = $juiste_code;


echo "<tr><td><b>Klik op dit plaatje:</b> <img src='images/$code_id.gif'></td></tr>";
echo"<tr><td>";
for($x = 0;$x < $aantal_codes;$x++)
{   
    ?> 
|<img src="images/<?=$x;?>.gif" border="0" style="cursor: pointer" onclick="forms[0].captcha.value='<?=$codes[$x];?>';forms[0].submit();" value="<?=$x?>">
    
    <?php    


}
?>