Share several ways to determine if a variable is empty in PHP_PHP Tutorial

1. isset
Function: Determine whether the variable has been initialized
Note: It does not determine whether the variable is empty, and can be used to determine whether the elements in the array have been defined
Note: When using isset is used to determine whether the array element has been initialized out of date. Its efficiency is about 4 times higher than array_key_exists

Copy code The code is as follows:

$a = '';
$a['c'] = '';
if (!isset($a)) echo '$a is not initialized' . "";
if ( !isset($b)) echo '$b has not been initialized' . "";
if (isset($a['c'])) echo '$a has been initialized' . "";
//The display result is
// $b has not been initialized
// $a has been initialized

2. empty
Function: detect whether the variable is "empty"

Note: Any uninitialized variable, a variable with a value of 0 or false or an empty string "" or null, an empty array, or an object without any attributes will be judged as empty==true
Note 1 : Uninitialized variables can also be detected as "empty" by empty

Note 2: empty can only detect variables, not statements

Copy code The code is as follows:

$a = 0;
$b = '';
$c = array();
if (empty($a)) echo '$a is empty' . "";
if (empty ($b)) echo '$b is empty' . "";
if (empty($c)) echo '$c is empty' . "";
if (empty($d)) echo '$d is empty' . "";

3. var == null
Function: Determine whether the variable is "null"
Description: The value is 0 or false or null character String "" or null variables and empty arrays will be judged as null

Note: The significant difference from empty is that var == null will report an error when the variable is not initialized.

Copy code The code is as follows:

$a = 0;
$b = array();
if ($a == null) echo '$a is empty' . "";
if ($b == null) echo '$b is empty' . "";
if ($c == null) echo '$b is empty' . "";
// The display result is
// $a is empty
// $b is empty
// Undefined variable: c

4. is_null
Function: Detect whether the variable is "null"
Description: When the variable is assigned a value of "null", the detection result is true

Note 1: null is not case sensitive: $a = null; $a = NULL makes no difference

Note 2: The detection result is true only when the value of the variable is "null", 0, empty string, false, and empty array are all detected as false

Note 3: When the variable is not initialized, the program will report an error

Copy the code The code is as follows:

$a = null;
$b = false;
if (is_null($a)) echo '$a is NULL' . "";
if (is_null($b)) echo '$b is NULL' . "";
if (is_null($c)) echo '$c is NULL' . "";
// The displayed result is
// $a is NULL
// Undefined variable : c

5. var === null
Function: Check whether the variable is "null", and the type of the variable must also be "null"
Description: When the variable is assigned a value When it is "null" and the type of the variable is also "null", the detection result is true

Note 1: When judging "null", equal to is_null has the same effect

Note 2: When the variable is not initialized, the program will report an error summary

In PHP, "NULL" and "empty" are two concepts. isset is mainly used to determine whether a variable has been initialized
empty can determine variables with values ​​of "false", "empty", "0", "NULL", and "uninitialized" as TRUE

is_null only evaluates variables with a value of "NULL" as TRUE

var == null Judges variables with values ​​of "false", "empty", "0", and "NULL" as TRUE

var === null Only variables with a value of "NULL" are judged as TRUE. Therefore, when we judge whether a variable is really "NULL", we mostly use is_null to avoid "false", "0" and other values. interference.

http://www.bkjia.com/PHPjc/328156.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/328156.htmlTechArticle1. isset function: determine whether the variable is initialized. Note: it does not determine whether the variable is empty, and can Used to determine whether the elements in the array have been defined. Note: When using isset to determine...