如何使用PHP实现商品评价与评论系统

<?php
$con = new PDO('mysql:host=localhost;dbname=mydb;charset=utf8', 'myuser', 'mypassWord');
$con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTioN);

//显示评论
if(isset($_GET['product_id'])){
 $stmt = $con->prepare('SELECT * FROM comments WHERE product_id=:product_id ORDER BY created_at DESC');
 $stmt->bindValue(':product_id', $_GET['product_id'], PDO::PARAM_INT);
 $stmt->execute();
 $comments = $stmt->fetchAll(PDO::FETCH_OBJ);
}else{
 $stmt = $con->query('SELECT * FROM comments ORDER BY created_at DESC');
 $comments = $stmt->fetchAll(PDO::FETCH_OBJ);
}

//显示添加评论的表格
echo '<fORM method="post" action="create_comment.php">';
echo '<h3>添加评论:</h3>';
echo '<p>用户名：<input type="text" name="username" required></p>';
echo '<p>评分：<input type="number" name="rating" min="1" max="5" required></p>';
echo '<p>评论：<textarea name="comment" required></textarea></p>';
echo '<input type="hidden" name="product_id" value="'.$_GET['product_id'].'">';
echo '<input type="submit" value="提交">';
echo '</form>';

//显示评论列表
echo '<h3>最新评论:</h3>';
foreach($comments as $comment){
 echo '<hr>';
 echo '<p>用户名：'.$comment->username.'</p>';
 echo '<p>评分：'.$comment->rating.'</p>';
 echo '<p>评论：'.$comment->comment.'</p>';
 echo '<p>发表时间：'.$comment->created_at.'</p>';
}
?>

<?php
if($_SERVER['REQUEST_METHOD'] !== 'POST'){
 header('Location: create_comments.php');
 exit;
}

$con = new PDO('mysql:host=localhost;dbname=mydb;charset=utf8', 'myuser', 'mypassword');
$con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

//验证表单数据
$stmt = $con->prepare('INSERT INTO comments (product_id, username, comment, rating) VALUES (:product_id, :username, :comment, :rating)');
$stmt->bindValue(':product_id', $_POST['product_id'], PDO::PARAM_INT);
$stmt->bindValue(':username', $_POST['username'], PDO::PARAM_STR);
$stmt->bindValue(':comment', $_POST['comment'], PDO::PARAM_STR);
$stmt->bindValue(':rating', $_POST['rating'], PDO::PARAM_INT);
$stmt->execute();

header('Location: create_comments.php?product_id='.$_POST['product_id']);
exit;
?>

<a href="create_comments.php?product_id=12345">添加评论</a>