PHP: Detailed explanation of null data type examples

What is a null value?

The null value, just like its name, means 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);
?>

Code running result:

#Second type:

is assigned the value NULL.

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

Code running results:

Third type:

Variables processed by the unset() function

<?php
$name = &#39;Felipe&#39;;

var_dump((unset) $name);
?>

Code running results:

Instance

String string1 is assigned null, string2 is not declared and assigned at all, so Outputs null. Although the last 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 code running result:

is_null function is used to determine whether the variable is null. We will talk about it later. Now First understand that this function returns a boolean type. If the variable is null, it returns true, otherwise it returns false. The unset() function is used to destroy known variables.

In the next section, we will explain "How to convert PHP data types".

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