Error: Notice: Undefined variable:

<?php // CAPTCHA instellen
ob_start();
srand ( microtime ( ) * 100000000 );
function gen_string ( $num_chars = 6 )
{
    $chars = 'a b c d e f g h i j k l m n o p q r s t u v w x y z 1 2 3 4 5 6 7 8 9 0';
    $chars = explode ( ' ', $chars );
    for ( $i = 0; $i < $num_chars; $i++ )
    {
        if ( strlen ( $return ) < $num_chars )  
        {
            $return .= $chars[rand ( 0, ( count ( $chars ) - 1 ) )];
        }
    }
    return $return;
}

// als het plaatje wordt opgehaald..
if ( isset ( $_GET['image'] ) )
{
// De instellingen..
    session_start ( );
    $code = ( $_SESSION['string'] ) ? base64_decode ( $_SESSION['string'] ) : 'ReLoAd';
    $lines = 7;
    $width = strlen ( $code ) * 20;
    $height = 30;
    $im = imagecreate ( $width, $height );
    
// De kleuren in het plaatje..

    $bg = imagecolorallocate ( $im, 0xff, 0xff, 0xff );
    $text_color = imagecolorallocate ( $im, 0x00, 0x00, 0x00 );
    $line_color = imagecolorallocate ( $im, 0xcc, 0xcc, 0xcc );
    imagecolortransparent ( $im );
    
// De lijnen in het plaatje..

    for ( $line = 0; $line < $lines; $line++ )
    {
        imageline ( $im, rand ( 0, $width ), rand ( 0, $height ), rand ( 0, $width ), rand ( 0, $height ), $line_color );
    }
    
    $chars = preg_split ( '//', $code );
    $x = -12;

    for ( $i = 0; $i < count ( $chars ); $i++ )
    {
        imagechar ( $im, rand ( 5, 5 ), $x, rand ( 0, $height - 15 ), $chars[$i], $text_color);
        $x = $x + 20;
    }

    if ( function_exists ( 'imagegif' ) )
    {
        header ( '(anti-spam-(anti-spam-(anti-spam-content-type:))) image/gif' );
        imagegif ( $im );
    }
    elseif ( function_exists ( 'imagejpeg' ) )
    {
        header ( '(anti-spam-(anti-spam-(anti-spam-content-type:))) image/jpeg' );
        imagejpeg ( $im );
    }
    imagedestroy ( $im );
}
else
{
    $characters = ( isset( $_GET['chars'] )) ? $_GET['chars'] : 6; 
    $_SESSION['string'] = base64_encode ( gen_string ( $characters ) );

}    ?>