sql 查询中为 foreach() 错误提供的参数无效

Question

所以我在 php 中开发分页系统，我使用 sql 从数据库中获取所有帖子，这是我的代码：

$sql = 'SELECT * FROM wp_posts WHERE post_parent="0" ORDER BY ID DESC LIMIT $start, $per_page';
$articles = $db->query($sql);

$start 应设置为 0，$per_page 应设置为 20

但是当运行结果通过foreach循环时returns这个警告：

Warning: Invalid argument supplied for foreach() in C:\wamp\www\System-tips\index.php on line 29 Call Stack

在 sql 查询中手动输入数字时效果很好，我做错了什么？

编辑：这里是完整的index.php代码

<?php
include"header.php";

//Delete Auto Draft posts
$del = 'DELETE FROM wp_posts WHERE post_title="Auto Draft"';
$db->query($del);

//Receive all posts
$sql = 'SELECT * FROM wp_posts WHERE post_parent="0" ORDER BY ID DESC';
$counter = $db->query($sql);

//Amount of pages
$row_cnt = $counter->num_rows;
$per_page = 5;
$pages = ceil($row_cnt/$per_page);

//Receive current page
if(!isset($_GET['page'])) {
header("location: index.php?page=1");
}
else {
$page = $_GET['page'];
}

$start = (($page - 1)*$per_page);
$sql = 'SELECT * FROM wp_posts WHERE post_parent="0" ORDER BY ID DESC LIMIT $start, $per_page';
$articles = $db->query($sql);

foreach ($articles as $article) {
echo'
<article>
<h3>'. $article['post_title'] .'</h3>';
search_picture($article['post_content']);
preview($article['post_content']);
echo '<a href="#">Read the article...</a>';
echo'</article>';

}

function search_picture($content) {
$array = explode('<img',$content);
if (isset($array[1])) {
$picture = explode(' />',$array[1]);
echo '<img' . $picture[0] . ' />'; }
echo '
<style>
img {
    width: 150px;
    height: 100px;
    float: left;
    margin-right: 20px;
}</style>
';
}

function preview($content) {
$preview = explode('<!--more-->',$content);
echo '<p style="margin-right: 20px;">' . $preview[0] . '</p>';
}

include"footer.php";
?>

Answer 1

我不确定这是否是你的问题，但你的字符串在这里：

'SELECT * FROM wp_posts WHERE post_parent="0" ORDER BY ID DESC LIMIT $start, $per_page'

... 包含在 ' 单引号中。 PHP 为 PHP，仅当字符串包含在 " 双引号中时，您在此处进行的插值才有效。因此，作为第一步，请尝试将 " 换成 '。

sql 查询中为 foreach() 错误提供的参数无效

Invalid argument supplied for foreach() mistake in sql query

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