ขอถามท่านผู้รู้ วิธีป้องกันการกรอก <script>alert('สวัสดี');</script>

function clean($input){
	$input = trim($input);
	if(get_magic_quotes_gpc()) {
		$input = stripslashes($input);
	}
	#ตัด html tag
	$input = strip_tags($input);
	return mysql_real_escape_string($input);
}

<?PHP echo clean("<script>alert('สวัสดี');</script>");?>

function clean($input){
	$input = trim($input);
	if(get_magic_quotes_gpc()) {
		$input = stripslashes($input);
	}
	$input = htmlspecialchars($input);
	return mysql_real_escape_string($input);
}

echo clean("<script>alert('สวัสดี');</script>");

htmlspecialchars();

echo htmlspecialchars("<script>alert('สวัสดี');</script>");

function html2txt($document){ 
$search = array('@<script[^>]*?>.*?</script>@si',  // Strip out javascript 
               '@<[\/\!]*?[^<>]*?>@si',            // Strip out HTML tags 
               '@<style[^>]*?>.*?</style>@siU',    // Strip style tags properly 
               '@<![\s\S]*?--[ \t\n\r]*>@'         // Strip multi-line comments including CDATA 
); 
$text = preg_replace($search, '', $document); 
return $text; 
}