如何解决PHP预处理语句报错Trying to access array offset on value of type null

问题原因 #

Trying to access array offset on value of type null 报错的核心原因是：当传入的$status在application表中没有匹配的记录时，fetch_row()方法会返回null，此时直接访问[0]数组偏移量就会触发对应报错。
你删除[0]后输出None，是因为此时$MostServiceRequested是数组类型，和字符串做相等判断永远不成立，最终会走到else分支。

修复方案 #

调整PHP代码，先判断查询是否返回有效结果再取值即可：

$stmt4 = $mysqli->prepare("SELECT ServiceRequested,status, COUNT(ServiceRequested) AS `occurrence` FROM application WHERE status = ? GROUP BY ServiceRequested ORDER BY `occurrence` DESC LIMIT 1");
$stmt4->bind_param('i',$status);
$stmt4->execute();
$result = $stmt4->get_result();
$row = $result->fetch_row();
// 先判空再取值
$MostServiceRequested = $row ? $row[0] : '';

if ($MostServiceRequested == 'ABC'){
    $ServiceRequested = 'ABC';
} elseif ($MostServiceRequested == 'DEF'){
    $ServiceRequested = 'DEF';
} elseif ($MostServiceRequested == 'GHI'){
    $ServiceRequested = 'GHI';
} else {
    $ServiceRequested = 'None';
}

原有前端输出代码无需修改：

<h3><?php echo $ServiceRequested; ?></h3>

可选优化 #

你可以把分支判断替换为数组匹配，代码更简洁易维护：

$validServices = ['ABC' => 'ABC', 'DEF' => 'DEF', 'GHI' => 'GHI'];
$ServiceRequested = $validServices[$MostServiceRequested] ?? 'None';

内容的提问来源于stack exchange，提问作者Laan