未定义是 php 中的数据类型吗? [英] Is undefined a data-type in php?

问题描述

undefined 是 php 中的数据类型吗?另外，如何检查它(在变量上，是否未定义)?

Is undefined a data-type in php? Also, how does one check for it (on a variable, is or is not undefined)?

推荐答案

PHP 中没有未定义"的数据类型.您可以检查使用 isset 设置的变量，但这无法区分根本没有设置的变量和它具有 null 值:

There is no "undefined" data type in PHP. You can check for a variable being set with isset, but this cannot distinguish between a variable not being set at all and it having a null value:

var_dump(isset($noSuchVariable)); // false

$nullVariable = null;
var_dump(isset($nullVariable)); // also false

但是，有一个技巧可以用于 compact 允许您确定变量是否已定义，即使其值为 null:

However, there is a trick you can use with compact that allows you to determine if a variable has been defined, even if its value is null:

var_dump(!!compact('noSuchVariable')); // false
var_dump(!!compact('nullVariable')); // true

现场示例.

isset 和 compact 技巧也同时适用于多个变量(使用逗号分隔的列表).

Both isset and the compact trick also work for multiple variables at once (use a comma-separated list).

在处理数组键时，您可以轻松区分 null 值和完全不存在:

You can easily distinguish between a null value and total absence when dealing with array keys:

$array = array('nullKey' => null);

var_dump(isset($array['nullKey'])); // false
var_dump(array_key_exists($array, 'nullKey')); // true

现场示例.

在处理对象属性时，还有 property_exists，相当于对象的array_key_exists.

When dealing with object properties there is also property_exists, which is the equivalent of array_key_exists for objects.

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