Several methods of judging variables in PHP to share _php skills

1. Isset
Function: To determine whether a variable is initialized
Description: 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 an array element is initialized, its efficiency is about 4 times times higher than array_key_exists

Copy Code code 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 '. "";
Display results as
$b has not been initialized
$a has been initialized

2. Empty
Function: Detect whether the variable is "null"

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

Note that 2:empty can only detect variables and not detect statements

Copy Code code 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 "null"
Description: A variable with a value of 0 or False or an empty string "" or null, an empty array, will be judged null

Note: The significant difference from empty is that var = null will error when the variable is uninitialized.

Copy Code code 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 '. "";
Display results as
$a is empty
$b is empty
Undefined Variable:c

4. Is_null
Function: Detects whether a variable is "null"
Note: The test result is true when the variable is assigned "null"

Note that 1:null is case-insensitive: $a = null; $a = NULL without any distinction

NOTE 2: Detection results are true,0, empty strings, false, empty arrays are detected as false only if the value of the variable is "null"

Note 3: The program will complain when the variable is not initialized

Copy Code code as follows:

$a = null;
$b = false;
if (Is_null ($a)) echo ' $a null '. "";
if (Is_null ($b)) echo ' $b null '. "";
if (Is_null ($c)) echo ' $c null '. "";
Display results as
$a is null
Undefined Variable:c

5. var = = null
Function: Detects whether a variable is "null" and the type of the variable must also be "null"
Note: When the variable is assigned "null" and the type of the variable is also "null", the test result is true

Note 1: On the judgment of "null", all equals the same effect as Is_null

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

in PHP, "null" and "NULL" are 2 concepts. Isset is primarily used to determine whether a variable has been initialized
Empty can judge a value of "false", "null", "0", "null", "uninitialized" as true

Is_null only evaluates to TRUE for a variable with a value of "null"

var = null evaluates to TRUE for variables with a value of "false", "null", "0", "null"

var = = null evaluates to TRUE only a variable with a value of "null" so when we judge whether a variable is really "null", most of them use is_null to avoid the interference of "false" or "0" equivalent.