"PHP" Determines whether the variable is controlled

1. isset function: Determine whether the variable is initialized

Description: It does not determine if the variable is empty and can be used to determine if the element in the array has been defined
Note: When using isset to determine if an array element is initialized, its efficiency is about 4 times times higher than the array_key_exists.

Copy CodeThe code is as follows:
$a = ";
$a [' c '] = ';
if (!isset ($a)) echo ' $a not initialized '. "";
if (!isset ($b)) echo ' $b not initialized '. "";
if (isset ($a [' C '])) echo ' $a has been initialized '. "";
Show results as
$b has not been initialized
$a has been initialized

2. Empty function: Detects if the variable is "empty"

Description: Any uninitialized variable, a value of 0 or FALSE, or an empty string "" or a null variable, an empty array, an object without any attributes, will be judged as Empty==true
Note 1: Uninitialized variables can also be detected as "empty" by empty
Note that 2:empty can only detect variables and not detect statements

Copy CodeThe 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 if the variable is "empty"

Description: A value of 0 or False or an empty string "" or a null variable, an empty array, will be judged null
Note: A significant difference from empty is that var = = NULL will be an error when the variable is not initialized.

Copy CodeThe 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 '. "";
Show results as
$a is empty
$b is empty
Undefined Variable:c

4. Is_null function: Detects if the variable is "null"

Description: The test result is true when the variable is assigned a value of "null"
Note 1:null is case insensitive: $a = null; $a = NULL without any difference
NOTE 2: Test results are true,0, empty strings, false, empty arrays are detected as false only when the value of the variable is "null"
Note 3: The program will error when the variable is not initialized

Copy CodeThe 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 '. "";
Show results as
$a is null
Undefined Variable:c

5. var = = = NULL Function: Detects if the variable is "null" and the type of the variable must also be "null"

Description: 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: On the judgment "null", all equals the same function as Is_null
NOTE 2: The program will error when the variable is not initialized

In summary php, "null" and "NULL" are 2 concepts.

Isset is used primarily to determine if a variable has been initialized.
Empty can evaluate to true for variables with a value of false, NULL, 0, NULL, uninitialized.
Is_null only variables with a value of "null" are judged as true
var = = null evaluates to "false", "null", "0", "null" for variables that are true
var = = NULL only evaluates a variable with a value of "null" to True

So when we judge whether a variable is really "NULL", most use Is_null to avoid "false", "0" equivalent interference.

"PHP" Determines whether the variable is controlled