Several methods for determining null variables in PHP _ PHP Tutorial

Several methods for judging null variables in PHP are shared. 1. isset: determines whether a variable is initialized. description: it does not determine whether the variable is null and can be used to determine whether the elements in the array are defined. Note: When isset is used, Judge 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.

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

2. empty
Function: checks 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.

The 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
Function: determines whether the variable is "null"
Note: variables and empty arrays whose values are 0, false, or null strings "or" null "are determined to be null.

Note: a significant difference from empty is that when the variable is not initialized, var = null will report an error.

The 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 value of the variable 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.

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 result is
// $ A is NULL
// Undefined variable: c

5. var = null
Function: checks 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: when the variable is not initialized, the program will report an error summary.

In PHP, "NULL" and "NULL" are two concepts. 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 with the value of "NULL" as TRUE.

Var = null. all variables with "false", "NULL", "0", and "null" are regarded as TRUE.

Var = null only determines whether a variable with the value of "NULL" is TRUE. Therefore, when determining whether a variable is actually "NULL", is_null is mostly used, in this way, the equivalent interference of "false" and "0" is avoided.

Http://www.bkjia.com/PHPjc/328156.htmlwww.bkjia.comtruehttp://www.bkjia.com/PHPjc/328156.htmlTechArticle1. isset: determines whether a variable is initialized. description: it does not determine whether the variable is null and can be used to determine whether the elements in the array are defined. Note: When isset is used to determine whether a variable is initialized...