php judgment variable

3. The difference between is_null(), empty(), isset()

//Special note: These three functions are only suitable for variable judgment, do not use literals directly

* 1. When is_null() returns true?

* 1. The variable has been declared but not initialized, and the default value is null

* 2. The variable is assigned a value of null

* 3. After unset() is destroyed, the variable has a null value

* Summary: If the variable does not exist/has no value assigned/the value is null, then true is returned

$val1;  //已声明,但未赋值
$val2 =  null; //直接用null初始化变量
$val3 = 'php';
unset($val3);  //彻底销毁变量

// Note: Using the ternary operator to output true or false is just for intuition and can be omitted completely

@var_dump(is_null($val1) ? true : false); //true
var_dump(is_null($val2) ? true : false); //true
@var_dump(is_null($val3) ? true : false); //true

// var_dump(is_null('')); //The empty string returns false

* 2. When does empty() return true?

* 1. Empty string, empty array

* 2. null

* 3.0 / '0' / false

* Summary:

* 1. If a variable does not exist, it is either empty or null, which can be determined using is_null() / empty()

* 2. If a variable exists, but its value has no impact on the running result, it is regarded as empty

* 3. Null must be empty, but empty is not necessarily null, because it may be a null value or 0 Or false

*/

$str1 = '';
$str2 = [];
$str3 = '0';
$str4 = 0;
$str5 = null;
$str6 = false;
$str7 = 'peter zhu';
echo &#39;<hr>&#39;;
var_dump(empty($str1) ? true : false);//空字符串
var_dump(empty($str2) ? true : false);//空数组
var_dump(empty($str3) ? true : false);//字符型数字0 
var_dump(empty($str4) ? true : false);//数字0
var_dump(empty($str5) ? true : false);//null值
var_dump(empty($str6) ? true : false);//布尔false
var_dump(empty($str7) ? true : false);//有值且不为空,返回false

* Thinking: To determine whether the user has entered content in the text box, should I use is_null() or empty()?

* Answer: Must Use empty(), not is_null()

* Reason: Because the value of the text box defaults to an empty string, that is, value = '', it has been assigned a value, it is just an empty value

* Therefore, is_null() can only check whether the value is null or not, and empty() must be used

* isset() is the inversion operation of null

* Summary: Variables Already exists, and its value is not null, returns true, otherwise false

$domain = 'www.php.cn';
$name = null;
$job;
echo &#39;<hr>&#39;;
var_dump(isset($var));
var_dump(isset($domain) ? true : false);
var_dump(isset($name) ? true : false);
var_dump(isset($job) ? true : false); //false,未赋值并不报错，与is_null不同

* Summary:

* 1. Variables have two states: declared, undeclared

* 2. Declared variables also have two states: assigned (initialized), unassigned (uninitialized)

* 3. Variables may be assigned value types: null, empty value, non-null Value

* 3.1: null value: is_null()

* 3.2: Empty value: empty()

* 3.3: Null value or non-empty value: isset()

* Basic usage principles:

* 1. For undeclared variables, only isset() can be used to judge

* 2. For declared variables, it is empty Use empty() to judge, and is_null() to judge whether to initialize

echo &#39;<hr color="red">&#39;;

//The first scenario: the variable is not declared

//$a is not declared

var_dump(isset($a) ? true : false);  //不报错,返回false表示未声明

/ /Example: When displaying data in paging, if there is a paging variable page in the current URL, the specified page will be output, otherwise the first page of data will be output by default

$name = isset($_GET['page']) ? $_GET['page'] : 1;

//is_null will give a warning and automatically Execute $a = null, so it will return true

var_dump(is_null($a) ? true : false);

//Because undeclared variables will be automatically initialized to null, and for variables with a value of null, empty() will consider them to be empty, so it will return true

var_dump(empty($a) ? true : false);

//Second scenario: The variable has been declared

$a = 'www.php.cn';
$b = '';
$c = null;
echo &#39;<hr color="blue">&#39;;
var_dump(isset($a));  //有无判断
var_dump(empty($b));  //非空判断
var_dump(is_null($c)); //null

//Principles for initializing variables:

//1. When the variable type is known

$num = 0; //数值
$userName = ''; //字符串
$isPass = false; //布尔
$books = []; //数组
$student = null; //对象

//2. Not sure what value the variable will eventually hold, it is recommended to initialize it with null