php 消息：尝试访问类型为 null 的值的数组偏移量
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Message: Trying to access array offset on value of type null
提问by vespino
I'm getting this error on multiple occasion in a script (invoiceplane) I have been using for a few years now but which hasn't been maintained unfortunately by its creators:
我在脚本（发票平面）中多次遇到此错误，我已经使用了几年，但不幸的是它的创建者没有维护它：
Message: Trying to access array offset on value of type null
My server has been upgrade to PHP 7.4 and I'm looking for a way to fix the issues and maintain the script myself since I'm very happy with it.
我的服务器已升级到 PHP 7.4，我正在寻找一种方法来解决问题并自己维护脚本，因为我对它非常满意。
This is what's on the line that gives the error:
这是给出错误的行上的内容：
$len = $cOTLdata['char_data'] === null ? 0 : count($cOTLdata['char_data']);
$cOTLdata is passed to the function:
$cOTLdata 传递给函数：
public function trimOTLdata(&$cOTLdata, $Left = true, $Right = true)
{
$len = $cOTLdata['char_data'] === null ? 0 : count($cOTLdata['char_data']);
$nLeft = 0;
$nRight = 0;
//etc
It's included in mpdfbtw, but simply overwriting the files from the github repository did not fix the errors.
顺便说一句，它包含在mpdf 中，但简单地覆盖 github 存储库中的文件并不能修复错误。
回答by ArSeN
This happens because $cOTLdatais not null but the index 'char_data'does not exist. Previous versions of PHP may have been less strict on such mistakes and silently swallowed the error / notice while 7.4 does not do this anymore.
发生这种情况是因为$cOTLdata它不为空但索引'char_data'不存在。以前版本的 PHP 可能对此类错误不那么严格，并且默默地吞下了错误/通知，而 7.4 不再这样做了。
To check whether the index exists or not you can use isset():
要检查索引是否存在，您可以使用isset()：
isset($cOTLdata['char_data'])
Which means the line should look something like this:
这意味着该行应如下所示：
$len = isset($cOTLdata['char_data']) ? count($cOTLdata['char_data']) : 0;
Note I switched the then and else cases of the ternary operator since === null is essentially what isset already does (but in the positive case).
注意我切换了三元运算符的 then 和 else 情况，因为 === null 本质上是 isset 已经做的（但在积极的情况下）。
