Summary of several methods to determine whether 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 to determine whether the array elements have been initialized, it is more efficient than array_key_exists. About 4 times

Copy code The code is as follows:

$a = '';
$a['c'] = ' ';
if (!isset($a)) echo '$a is not initialized' . "";
if (!isset($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 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 "empty"

Note: Variables and empty arrays with a value of 0 or false or empty string "" or null will be judged as null
Note: The significant difference from empty is: when the variable is not initialized, var == null will An error will be reported.

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"

Note: When the variable is assigned the value "null", the detection result is true
Note 1: null is not case-sensitive: $a = null; $a = NULL makes no difference
Note 2: Only 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 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: detect whether the variable is "null", and the type of the variable must also be "null"

Note: When a variable is assigned a value of "null" and the type of the variable is also "null", the detection result is true
Note 1: When it is judged to be "null", equal to is_null has the same effect
Note 2: When the variable is not initialized, the program will report an error

To summarize, "NULL" and "empty" are two concepts in PHP.

isset is mainly used to determine whether the variable has been initialized
empty. Variables with values ​​​​of "false", "empty", "0", "NULL", and "uninitialized" can be determined as TRUE
is_null Only variables with a value of "NULL" are judged as TRUE
var == null Variables with a value of "false", "empty", "0", and "NULL" are judged as TRUE
var == = null Only variables with a value of "NULL" are judged as TRUE

So when we judge whether a variable is really "NULL", we mostly use is_null to avoid interference from "false", "0" and other values.

http://www.bkjia.com/PHPjc/825194.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/825194.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...