为什么我的变量没有定义？

Question

我注意到这是php中的一个常见错误，我试图在网上搜索，但找不到答案，我正在尝试创建一个simpe论坛，我得到了这些错误

Notice: Undefined variable: topics in C:\xampp\htdocs\mysite\Students forum\view_category.php on line 83

Notice: Undefined variable: topic in C:\xampp\htdocs\mysite\Students forum\view_category.php on line 86

这是第83行的代码

$topics .= "<table width='100%' style='border-collapse: collapse;'>";

这是第86行的代码

$topic .= "<tr><td colspan='4'><hr /></td></tr>";

这是我的代码

if (mysql_num_rows($res2) > 0) {
        // Appending table data to the $topics variable for output on the page
        $topics .= "<table width='100%' style='border-collapse: collapse;'>";
        $topics .= "<tr><td colspan='4'><a href='index.php'>Return To Forum Index</a>".$logged."<hr /></td></tr>";
        $topics .= "<tr style='background-color: #dddddd;'><td>Topic Title</td><td width='65' align='center'>Last User</td><td width='65' align='center'>Replies</td><td width='65' align='center'>Views</td></tr>";
        $topic .= "<tr><td colspan='4'><hr /></td></tr>";
        // Fetching topic data from the database
        while ($row = mysql_fetch_assoc($res2)) {
            // Assign local variables from the database data
            $tid = $row['id'];
            $title = $row['topic_title'];
            $views = $row['topic_views'];
            $date = $row['topic_date'];
            $creator = $row['topic_creator'];
            // Check to see if the topic has every been replied to
            if ($row['topic_last_user'] == "") { $last_user = "N/A"; } else { $last_user = getusername($row['topic_last_user']); }
            // Append the actual topic data to the $topics variable
            $topics .= "<tr><td><a href='view_topic.php?cid=".$cid."&tid=".$tid."'>".$title."</a><br /><span class='post_info'>Posted by: ".getusername($creator)." on ".convertdate($date)."</span></td><td align='center'>".$last_user."</td><td align='center'>".topic_replies($cid, $tid)."</td><td align='center'>".$views."</td></tr>";
            $topics .= "<tr><td colspan='4'><hr /></td></tr>";
        }
        $topics .= "</table>";
        // Displaying the $topics variable on the page
        echo $topics;
    } 

Answer 1

运算符.=通常用于连接右参数和左参数，但在本例中，您没有左参数。必须按如下方式实例化该变量：

$topics = "<table width='100%' style='border-collapse: collapse;'>";
$topic = "<tr><td colspan='4'><hr /></td></tr>";

if( ... ) {
    while( ... ) {
        $topics .= ...
        $topic .= ...
    }
}

Answer 2

在使用$topics .="anything"之前首先声明变量

$topics =''; 
$topics .= "whatever you want"; // must have been declared before

所以在这种情况下，它可以像这样

if (mysql_num_rows($res2) > 0) {
    $topics="<table width='100%' style='border-collapse: collapse;'>"
    $topics .= "<tr><td colspan='4'><hr /></td></tr>";
    // rest of your code
}

Answer 3

此$topics .=是$topics = $topics .的简写，如果您第一次使用变量时使用它，则该变量不会被视为“已定义”。

所以你至少应该这样做：

$topics = '';
$topic = '';

在你开始使用速记之前。