PHP determines whether the array is empty.

PHP determines whether the array is empty.

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.

 
 

  
  
<? Php
  
  
$ 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 with a value of 0 or false or an empty string "or a null variable, an empty array, and an 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.

 
 

  
  
<? Php
  
  
$ A = 0;
  
  
$ B = '';
  
  
$ C = array ();
  
  
If (emptyempty ($ a) echo '$ a is blank '."";
  
  
If (emptyempty ($ B) echo '$ B is blank '."";
  
  
If (emptyempty ($ c) echo '$ c is blank '."";
  
  
If (emptyempty ($ d) echo '$ d is blank '."";
 
 

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

Description: variables and empty arrays whose values are 0, false, or empty 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.

 
 

  
  
<? Php
  
  
$ 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 detection 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.

 
 

  
  
<? Php
  
  
$ 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: The program reports an error when the variable is not initialized.

Summary:

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

Isset is mainly used to determine whether a variable has been initialized.

Empty can judge TRUE for all variables whose values are "false", "NULL", "0", "NULL", and "uninitialized ".

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 the variable with the value of "NULL" as TRUE.

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