php SQL Injection with MySQL

<form method="GET" action="search.php" name="search">
<input name="keywords" type="text" value="" size="15"> <input type="submit" value="Search">
</form>
<p><b>Search result</b></p>

<?php
$servername = "localhost";
$dbusername = "root";
$dbpassword = "";
$dbname = "injection";

mysql_connect($servername,$dbusername,$dbpassword) or die ("数据库连接失败");

$keywords = $_GET['keywords'];
if (!empty($keywords)) {
　　//$keywords = addslashes($keywords);
　　//$keywords = str_replace("_","_",$keywords);
　　//$keywords = str_replace("%","%",$keywords);

　　$sql = "SELECT * FROM ".$db_prefix."article WHERE title LIKE '%$keywords%' $search ORDER BY title DESC";
　　$result = mysql_db_query($dbname,$sql);
　　$tatol=mysql_num_rows($result);

　　echo "<p>SQL Query:$sql<p>";

　　if ($tatol <=0){
　　　　echo "The "<b>$keywords</b>" was not found in all the record.<p>n";
　　} else {
　　　　while ($article=mysql_fetch_array($result)) {
　　　　　　echo "<li>".htmlspecialchars($article[title])."<p>n";
　　　　} //while
　　}
} else {
　　echo "<b>Please enter some keywords.</b><p>n";
}
?>

%' ORDER BY articleid/*
%' ORDER BY articleid#
__' ORDER BY articleid/*
__' ORDER BY articleid#

SELECT * FROM article WHERE title LIKE '%%' ORDER BY articleid/*%' ORDER BY title DESC
SELECT * FROM article WHERE title LIKE '%__' ORDER BY articleid#%' ORDER BY title DESC