php 判断变量为null_PHP中的NULL | 如何设置和检查NULL变量？

心已赠人 2023-03-05 11:56 19阅读 0赞

php 判断变量为null

## PHP中为NULL **(**NULL in PHP**)** ##

**NULL** is a special value in PHP, it represents that a variable does contain any value or a variable is undefined. Undefined variable or variable with no value is a type of null.

**NULL**是PHP中的一个特殊值，它表示变量确实包含任何值或变量未定义。 未定义的变量或没有值的变量是null的类型。

If a variable is assigned with a **NULL** value, or if any variable is not assigned any value or if a variable is unset using unset() function – variable will be considered as null in PHP.

如果为变量分配了**NULL**值，或者没有给变量分配任何值，或者使用unset()函数未设置变量，则在PHP中变量将被视为null 。

**Checking whether a variable contains NULL or not?**

**检查变量是否包含NULL？**

To check whether a variable contains a **NULL** value or not, we use is\_null() function, it returns true (1), if a variable contains a **NULL** value or if a variable is undefined.

为了检查变量是否包含**NULL**值，我们使用is\_null()函数，如果变量包含**NULL**值或变量未定义，则返回true (1)。

**Note:** If the variable does not has any value or unset using by unset() function, PHP returns a notice that **"Undefined variable"**

**注意：**如果变量不具有任何值或未使用unset()函数设置 ，则PHP返回通知**“ Undefined variable”**

**Example:**

**例：**

Input:
        $var = NULL;
    
        Function call:
        is_null($var);
    
        Output:
        1

**PHP code to demonstrate use of NULL, is\_null() and unset()**

**PHP代码演示NULL，is\_null()和unset()的用法**

<?php
        $var1 = "Hello";
        $var2 = NULL;
        $var3 = 10;
        $var4 = 20;
        
        if(is_null($var1))  print("var1 contains NULL\n");
        else print("var1 contains $var1\n");
    
        if(is_null($var2)) print("var2 contains NULL\n");
        else print("var2 contains $var2\n");
    
        if(is_null($var3)) print("var3 contains NULL\n");
        else print("var3 contains $var3\n");
    
        if(is_null($var4)) print("var4 contains NULL\n");
        else print("var4 contains $var4\n");        
        
        //unsetting all variables
        unset($var1);
        unset($var2);
        unset($var3);
        unset($var4);
    
        if(is_null($var1))  print("var1 contains NULL\n");
        else print("var1 contains $var1\n");
    
        if(is_null($var2)) print("var2 contains NULL\n");
        else print("var2 contains $var2\n");
    
        if(is_null($var3)) print("var3 contains NULL\n");
        else print("var3 contains $var3\n");
    
        if(is_null($var4)) print("var4 contains NULL\n");
        else print("var4 contains $var4\n");            
    ?>

**Output**

**输出量**

var1 contains Hello
    var2 contains NULL
    var3 contains 10
    var4 contains 20
    PHP Notice:  Undefined variable: var1 in /home/main.php on line 25
    var1 contains NULL
    PHP Notice:  Undefined variable: var2 in /home/main.php on line 28
    var2 contains NULL
    PHP Notice:  Undefined variable: var3 in /home/main.php on line 31
    var3 contains NULL
    PHP Notice:  Undefined variable: var4 in /home/main.php on line 34
    var4 contains NULL

> 翻译自: [https://www.includehelp.com/php/null.aspx][https_www.includehelp.com_php_null.aspx]

php 判断变量为null

[https_www.includehelp.com_php_null.aspx]: https://www.includehelp.com/php/null.aspx