Let's talk about the concepts and differences between null and empty in PHP

In PHP, null and empty are two very common and easily confused concepts. Although they look similar, they have different meanings and usage. In this article, we will introduce the concept and difference between null and empty.

1. The meaning and use of null

null is a special type that represents "non-value". It indicates that a variable has not been assigned a value or has been assigned a value of null. When a variable is assigned null, it no longer has any value. Here is an example:

$var = null;

In this example, the $var variable is assigned null, which means it no longer has any value. If you try to print the value of $var, you'll get a null value (i.e. you won't get any output).

null can be used in many situations, such as:

• When initializing a variable;
• Checking the return value of a function;
• Variable dereference.

Here are some examples:

$name = null; // 初始化变量

function getUser($id) {
  if($id > 0) {
    // 假设查询到了一个用户
    return $user;
  } else {
    // 如果 id 小于等于 0，返回 null
    return null;
  }
}

$user = getUser($id); // 检查函数返回值

$person = new stdClass();
$person->name = 'John';
$person = null; // 取消引用变量

2. The meaning and use of empty (empty)

Different from null, empty (empty) is A concept that represents a "null value" rather than a data type in a programming language. A null value can be any of the following situations:

• A variable is assigned an empty string ('');
• A variable is assigned a value of 0 or 0.0;
• A variable is assigned a value of false;
• A variable is declared but not assigned a value.

Here are some examples:

$var1 = ''; // 空字符串
$var2 = 0; // 0
$var3 = 0.0; // 0.0
$var4 = false; // false
$var5; // 未赋值，默认为 null，但在条件语句中会被视为 empty

Null values ​​are often used to check whether a variable has a value. For example, you might check whether the user entered a value into a field when they submitted a form. Here is an example:

if(empty($_POST['username'])) {
  echo '请输入用户名';
}

In this example, we check if $_POST['username'] is null. If so, output a message prompting the user to enter a username.

3. The difference between null and empty

Although null and empty may look similar, there are some important differences between them. Here are some differences:

• null means it does not exist or has not been initialized, while empty means it has been initialized but does not contain any value.
• null is a separate data type, while empty is not.
• Logically speaking, null and empty represent different meanings. null means you don't know what the value is, while an empty value means you know what the value is but nothing.
• empty contains null in the conditional statement. That is, if a variable is considered empty, it is also null.

4. Summary

In PHP, null and empty are two very common and easily confused concepts. Although they look very similar, they have different meanings and usage. null means that a variable has not been assigned a value or has been assigned a value of null, while a null value means that a variable has been assigned a value but does not contain any value.

When using it, we need to choose to use null or empty value according to different scenarios. If we want to check whether a variable has been initialized, we should usually use empty. If we want to check whether the return value of a variable is valid, we should usually use null.

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