PHP中如何从空MySQL游标创建默认变量避免报错

问题根源是当查询无匹配行时，$row_c = $stmt_c->fetch(PDO::FETCH_OBJ)会返回null，此时在else分支直接给$row_c->subject等属性赋值，相当于试图给空值创建默认对象，因此触发警告。

核心解决方法 #

先将$row_c初始化为一个空的stdClass对象，再根据查询结果覆盖属性：

// 初始化空对象，预设默认值
$row_c = new stdClass();
$row_c->subject = "";
$row_c->textcontent = "";
$row_c->notes = "";

$query_c = "SELECT * FROM content WHERE `cid` = ".$_SESSION['cid'];
$stmt_c = $dbh->query($query_c);
$fetched_row = $stmt_c->fetch(PDO::FETCH_OBJ);

// 查询到结果时，覆盖默认值
if ($fetched_row) {
  $row_c->subject = str_replace("`","'",$fetched_row->subject);
  $row_c->textcontent = str_replace("`","'",$fetched_row->textcontent);
  $row_c->notes = str_replace("`","'",$fetched_row->notes);
}

额外优化建议 #

1. 原代码中$rowcount变量未定义，直接用$fetched_row是否为真判断即可（fetch返回false/null表示无结果）。
2. 直接拼接$_SESSION['cid']存在SQL注入风险，建议改用预处理语句：

$row_c = new stdClass();
$row_c->subject = "";
$row_c->textcontent = "";
$row_c->notes = "";

// 使用预处理语句避免SQL注入
$query_c = "SELECT * FROM content WHERE `cid` = ?";
$stmt_c = $dbh->prepare($query_c);
$stmt_c->execute([$_SESSION['cid']]);
$fetched_row = $stmt_c->fetch(PDO::FETCH_OBJ);

if ($fetched_row) {
  $row_c->subject = str_replace("`","'",$fetched_row->subject);
  $row_c->textcontent = str_replace("`","'",$fetched_row->textcontent);
  $row_c->notes = str_replace("`","'",$fetched_row->notes);
}

内容的提问来源于stack exchange，提问作者jack.linton