Warning: pack() [function.pack]: Type H: illegal hex digit in C:\AppServ\www\

pack('H', '0123456789ABCDEF'); // OK
pack('H', 'Hello World!!!'); // Error

<?
$host="localhost"; 
$username="root"; 
$pass_word="pp34999"; 
$db="monitora"; 
$Conn = mysql_connect( $host,$username,$pass_word) or die ("connect error");
mysql_query("SET NAMES utf8",$Conn); 
mysql_select_db($db) or die("db error"); 
mysql_query("SET character_set_results=tis620");
mysql_query("SET character_set_client=tis620");
mysql_query("SET character_set_connection=tis620");
$sql = "select * from friday WHERE ID = '207'"; 
$result = mysql_query($sql) or die(mysql_error());
while($row = mysql_fetch_array($result))
{
$image = "$row[rfid]";
echo "$image <br/>";
}

$hexadecimal_string ="$image";
$ss = base64_encode(pack('H*',$hexadecimal_string));
echo "$ss <br/>";

$data="$ss";
echo '<img src="data:image/gif;base64,' . $data. '" />';
?>

if (preg_match('/[^0-9a-fA-F]/', $hexadecimal_string, $matches, PREG_OFFSET_CAPTURE)) {
	print_r($matches);
	echo "Invalid char at offset $matches[1]<br />";
}