What does null value mean in php

php Null value refers to the null data type, which means that a variable has no value; null value (null) is not case-sensitive, that is, null is the same as NULL or Null.

The operating environment of this article: windows7 system, PHP7.1 version, DELL G3 computer

php What does the null value mean?

php: Null data type

Null value:

Null value, just like its name , indicating that a variable has no value. The null value (null) is not case-sensitive, that is, null is the same as NULL or Null.

There are three situations in which a null value is assigned in PHP:

The first one:

has not been assigned a value

<?php
$a;
var_dump($a);
?>

Second type:

is assigned NULL.

<?php
$nu=null;
var_dump($nu);
?>

Third type:

Variables processed by the unset() function

<?php
$name = &#39;Felipe&#39;;
 
var_dump((unset) $name);
?>

Example

String string1 is assigned null, string2 has no declaration or assignment at all, so null is output. Although the final string3 is assigned an initial value, it also becomes null after being processed by the unset() function. The function of unset() function is to delete variables from memory. The code is as follows:

<?php
header("content-type:text/html;charset=utf-8");
echo  "变量($string1)直接赋值为null";
$string=null;                                          //$string1被赋空值
$string3="str";                                      //$string3被赋值str
if(is_null($strig1=null)){                    //判断$string1是否为空
    echo &#39;string=null&#39;;
}
echo &#39;<p>&#39;;
echo &#39;变量($string2)没有被赋值&#39;;
if(is_null($string2=null)){                 //判断$string1是否为空
    echo &#39;string=null&#39;;
}
echo &#39;<p>&#39;;
echo &#39;被unset()释放过的变量($string3)：&#39;;
unset($string3);
if(is_null($string3=null)){//判断$string1是否为空
    echo &#39;string=null&#39;;
}
?>

The is_null function is used to determine whether the variable is null. We will talk about it later. Let’s understand it first. This function returns a boolean type. If the variable is null , returns true, otherwise returns false. The unset() function is used to destroy known variables.

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