How to determine if a variable is empty in php

The judgment variable is empty, which is used on many occasions. At the same time, like many novices, I often make some mistakes, so I sorted out some commonly used judgment variables in PHP. Empty method.

1. isset function: determine whether the variable is initialized

Note: It does not determine whether the variable is empty, and can be used to determine whether the elements in the array are initialized. defined.

Note: When using isset to determine whether an array element has been initialized, its efficiency is about 4 times higher than array_key_exists.

$a = '';

$a['c'] = '';

if (!isset($a)) echo '$a has not been Initialized' . "";

if (!isset($b)) echo '$b is not initialized' . "";

if (isset($a['c'] )) echo '$a has been initialized' . "";

// The display result is

// $b has not been initialized

// $a has been Initialization

Related recommendations: "php Getting Started Tutorial"

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

Description: Any uninitialized variable, variable with value 0 or false or empty string "" or null, empty array, or 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.

$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: Variables and empty arrays with a value of 0 or false or an empty string "" or null 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.

$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 displayed 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.

$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 display result is

// $a is NULL

// Undefined variable: c

5. var === null function: detection Whether the variable is "null", and the type of the variable must also be "null"

Explanation: When the variable is assigned a value of "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 set variables with values ​​of "false", "empty", "0", "NULL", and "uninitialized" Judged as TRUE

is_null Only variables with a value of "NULL" are judged as TRUE

var == null and values ​​are "false", "empty", "0", and "NULL" All variables are judged to be TRUE

var === null. Only variables with a value of "NULL" are judged to be TRUE

. Therefore, when we judge whether a variable is really "NULL", most of the Use is_null to avoid interference with values ​​such as "false" and "0".

6. Determine whether there is a null value in the obtained POST array:

[codes]$data = $_POST;

foreach ($data as $value){

if($value == ""){

error('Please fill in the form completely');

}

}[/codes]

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