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

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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：

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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

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<?php
$var1 = "Hello";
$var2 = NULL;
$var3 = 10;
$var4 = 20;
if(is_null($var1))
print("var1 contains NULLn");
else print("var1 contains $var1n");
if(is_null($var2)) print("var2 contains NULLn");
else print("var2 contains $var2n");
if(is_null($var3)) print("var3 contains NULLn");
else print("var3 contains $var3n");
if(is_null($var4)) print("var4 contains NULLn");
else print("var4 contains $var4n");
//unsetting all variables
unset($var1);
unset($var2);
unset($var3);
unset($var4);
if(is_null($var1))
print("var1 contains NULLn");
else print("var1 contains $var1n");
if(is_null($var2)) print("var2 contains NULLn");
else print("var2 contains $var2n");
if(is_null($var3)) print("var3 contains NULLn");
else print("var3 contains $var3n");
if(is_null($var4)) print("var4 contains NULLn");
else print("var4 contains $var4n");
?>

Output

输出量

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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