Usage and differences of empty,isset,is_null in PHP

I was reading the source code of the project recently and found that the empty, isset and is_null functions (language features) are used indiscriminately in the source code. In some places, it is obvious that there are pitfalls. Failure to understand these things correctly may create pitfalls for subsequent development.

1.empty usage

bool empty (mixed var)
If var is a non-empty or non-zero value, empty() returns FALSE . In other words, "", 0, "0", NULL, FALSE, array(), var $var; and objects without any attributes will be considered empty. If var is empty, TRUE is returned

2.isset()

isset -- Check whether the variable is set

Description
bool isset ( mixed var [, mixed var [, ...]])
Returns TRUE if var exists, otherwise returns FALSE.
If a variable has been released using unset(), it will no longer be isset(). If you use isset() to test a variable that is set to NULL, it will return FALSE. Also note that a NULL byte ("0") is not equivalent to PHP's NULL constant.

Note: If the variable does not exist, neither isset() nor empty() will report an error; is_null(), is_numeric() will report an error

How to distinguish [0,'' in the following array ,null] Three elements? (1) Difference 0:

$a = 0;
isset($a) && is_numeric($a) === true

(2) Difference ''

$a = '';
empty($a) && $a=== ''

(3) Difference between null

$a = null;
is_null($a);

In addition, when submitting a form, you may often need to check whether a variable exists, if $_REQUEST['status'] = 0 ;Use empty($_REQUEST['status']) to return true, but use isset($_REQUEST['status']) to judge that it is not empty

3. is_null():

bool is_null (mixed $var) (function definition in php.net official document)
When the parameters meet the following three conditions, is_null() will return TRUE, and in other cases it will be FALSE
1. It is assigned a value of NULL
2. It has not been assigned a value
3. It is undefined, which is equivalent to unset(). After unset() a variable, it is not defined.
Let Let’s take a look at some examples:

$myvar = NULL;  
var_dump(is_null($myvar)); // TRUE 
$myvar1;      
var_dump(is_null($myvar1)); // TRUE Notice: Undefined variable 
$num = 520; 
unset($num); 
var_dump(is_null($num)); //TRUE Notice: Undefined variable 
var_dump(is_null($some_undefined_var)); //TRUE Notice: Undefined variable 
 
$myvar = 0; is_null($myvar);   // FALSE 
$myvar = FALSE; is_null($myvar); // FALSE 
$myvar = ''; is_null($myvar);  // FALSE

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