索引空变量时未引发php undefined索引通知 [英] php undefined index notice not raised when indexing null variable

问题描述

我很想知道PHP是否打算执行以下行为.并且，如果有意的话，是否可以通过在空变量中创建索引来初始化数组(如第一个代码段中所示)?

I'm curious to know if the following behaviour in PHP is intended or not. And, if it is intended, it is considered acceptable to initialize an array from a null variable by creating an index into it (as is done in the first code snippet)?

error_reporting(E_ALL);
$arr = null;

echo ($arr["blah"]===null) ? "null" : $arr["blah"];

$arr["blah"] = "somevalue";
echo "<br>";
echo ($arr["blah"]===null) ? "null" : $arr["blah"];
var_dump ($arr);

此输出

null
somevalue

array (size=1)
   'blah' => string 'somevalue' (length=9)

但是，如果首先初始化数组(请参见下面的代码)，则会得到完全相同的输出，但是当我第一次尝试 $ arr ["blah"]

However, if the array is initialized first (see code below), I get the exact same output, but an "Undefined Index" notice is given when I first try $arr["blah"]

error_reporting(E_ALL);
$arr = array();

echo ($arr["blah"]===null) ? "null" : $arr["blah"];

$arr["blah"] = "somevalue";
echo "<br>";
echo ($arr["blah"]===null) ? "null" : $arr["blah"];
var_dump ($arr);

推荐答案

如果数组为null，PHP将不会尝试进行比较.

PHP won't attempt the comparison if the array is null.

在第二种情况下，因为设置了数组，所以确实发生了比较.PHP不会检查它是否为空.

In the second circumstance, a comparison does occur because the array is set. PHP does not check to see if it is empty.

您的三元尝试访问变量$ arr ["blah"]，而不在进行比较之前不检查变量是否已设置.

Your ternary is attempting to access the variable $arr["blah"], not checking to see if it is set before doing a comparison.

正确的写法是:

error_reporting(E_ALL);
$arr = array();

if(isset($arr["blah"])) echo ($arr["blah"]===null) ? "null" : $arr["blah"];

$arr["blah"] = "somevalue";
echo "<br>";
if(isset($arr["blah"])) echo ($arr["blah"]===null) ? "null" : $arr["blah"];
var_dump ($arr);

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