Summary of examples of methods to determine whether a variable is empty in PHP

JudgmentVariable 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 that determine whether the variable is empty. Method

1. isset function: Determine whether the variable is initialized

Description: It does not determine whether the variable is empty, and can be used to determine whether the array Whether the elements in
have been definedNote: When using isset to determine whether the array elements have been initialized, its efficiency is about 4 times higher than array_key_exists

$a = '';
$a['c'] = '';
if (!isset($a)) echo '$a 未被初始化' . "";
if (!isset($b)) echo '$b 未被初始化' . "";
if (isset($a['c'])) echo '$a 已经被初始化' . "";
// 显示结果为
// $b 未被初始化
// $a 已经被初始化

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

Description: Any uninitialized variable, the value is 0 or false or emptyString"" or null Variables, empty arrays, and objects 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 '$a 为空' . "";
if (empty($b)) echo '$b 为空' . "";
if (empty($c)) echo '$c 为空' . "";
if (empty($d)) echo '$d 为空' . "";

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 report an error .

$a = 0;
$b = array();
if ($a == null) echo '$a 为空' . "";
if ($b == null) echo '$b 为空' . "";
if ($c == null) echo '$b 为空' . "";
// 显示结果为
// $a 为空
// $b 为空
// 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 There is no difference
Note 2: The detection result is true only when the value of the variable is "null", 0, empty string, Both false and empty arrays are 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 '$a 为NULL' . "";
if (is_null($b)) echo '$b 为NULL' . "";
if (is_null($c)) echo '$c 为NULL' . "";
// 显示结果为
// $a 为NULL
// Undefined variable: c

5. var === null function: detect whether the variable is "null", At the same time, the type of the variable must also be "null"

Note: 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 it is judged as "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 2 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 Judged as TRUE are variables with values ​​of "false", "empty", "0", and "NULL"

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 "false", Interference equivalent to "0". ###

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