Summary of several methods for determining null variables in PHP

1. isset function: determines whether a variable is initialized.

Note: It does not determine whether the variable is null and can be used to determine whether the elements in the array have been defined.
Note: When isset is used to determine whether the array element is initialized or not, it is about 4 times more efficient than array_key_exists.

Copy codeThe 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 result is
// $ B is not initialized
// $ A has been initialized

2. empty function: Check whether the variable is "null"

Note: Any uninitialized variable, variable with a value of 0 or false or null String "or null, empty array, and object without any attribute, empty = true
Note 1: uninitialized variables can also be detected as "null" by empty"
NOTE 2: empty can only detect variables, but cannot detect statements.

Copy codeThe Code is as follows:
$ A = 0;
$ B = '';
$ C = array ();
If (empty ($ a) echo '$ a is blank '."";
If (empty ($ B) echo '$ B is blank '."";
If (empty ($ c) echo '$ c is blank '."";
If (empty ($ d) echo '$ d is blank '."";

3. var = null: determines whether the variable is "null"

Note: variables and empty arrays whose values are 0, false, or empty strings "" or null will all be considered null.
Note: A significant difference from empty is that when the variable is not initialized, var = null will report an error.

Copy codeThe Code is as follows:
$ A = 0;
$ B = array ();
If ($ a = null) echo '$ a is blank '."";
If ($ B = null) echo '$ B is blank '."";
If ($ c = null) echo '$ B is blank '."";
// The result is
// $ A is empty
// $ B is empty
// Undefined variable: c

4. is_null function: checks whether the variable is "null"

Note: When the variable is assigned "null", the detection result is true.
Note 1: null is case insensitive: $ a = null; $ a = NULL no difference
Note 2: The check result is true only when the variable value is "null". The values 0, null String, false, and empty array are both false.
Note 3: The program reports an 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 '."";
// The result is
// $ A is NULL
// Undefined variable: c

5. var = null: Check whether the variable is "null" and the variable type must be "null"

Note: When a variable is assigned "null" and the variable type is "null", the detection result is true.
Note 1: when it is determined to be "null", all functions are the same as is_null.
Note 2: The program reports an error when the variable is not initialized.

In summary, "NULL" and "NULL" are two concepts in PHP.

Isset is mainly used to determine whether a variable has been initialized.
Empty can be used to set TRUE values for "false", "NULL", "0", "NULL", and "uninitialized" variables.
Is_null only determines the variable whose value is "NULL" as TRUE.
Var = null. All variables with false values, empty values, 0 values, and NULL values are regarded as TRUE.
Var = null only determines the variable with the value of "NULL" as TRUE.

Therefore, when determining whether a variable is "NULL", is_null is mostly used to avoid interference equivalent to "false" and "0.