What is the difference between the use of isset function and empty function in php? (comprehensive)

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PHP’s isset() function is generally used to detect whether a variable is set
Format: bool isset (mixed var [, mixed var [, ...]] )

Function: Check whether the variable is set

Return value:

If the variable does not exist, return FALSE
If the variable exists and its value is NULL, also returns FALSE
If the variable exists and the value is not NULL, TRUE is returned
When checking multiple variables at the same time, TRUE will be returned only when each single item meets the previous requirement, otherwise the result is FALSE
Versions: PHP 3, PHP 4, PHP 5
More instructions:
After using unset() to release a variable, it will no longer be isset().
PHP function isset() can only be used for variables. Passing any other parameters will cause a parsing error.
To detect whether a constant has been set, use the defined() function.

PHP's empty() function determines whether the value is empty

Format: bool empty (mixed var)

Function: Check whether a variable is empty

Return value:

If the variable does not exist, return TRUE
If the variable exists and its value is "", 0 , "0", NULL,, FALSE, array(), var $var; and objects without any attributes, return TRUE
If the variable exists and the value is not "", 0, "0", NULL,, FALSE, array(), var $var; and objects without any attributes, return FALSE
Versions: PHP 3, PHP 4, PHP 5
More instructions:
The return value of empty() = !(boolean) var, but no warning message will be generated because the variable is undefined. See Converting to Boolean for more information.
empty() can only be used for variables. Passing any other parameters will cause a Paser error and terminate the operation.
To detect whether a constant has been set, use the defined() function.

Example: A simple comparison of empty() and isset()

<?php 
$var = 0; 
// 结果为 true，因为 $var 为空 
if (empty($var)) { 
echo &#39;$var is either 0 or not set at all&#39;; 
} 
// 结果为 false，因为 $var 已设置 
if (!isset($var)) { 
echo &#39;$var is not set at all&#39;; 
} 
?>

Note: Since this is a language structure rather than a function, it cannot Called by variable function.
Note: empty() only detects variables, and detecting anything that is not a variable will result in a parsing error. In other words, the following statement will not work: empty(addslashes($name)).
The following is a detailed example code of isset and empty functions that has been tested by Script House. After reading this, it is basically the same:

<?php 
error_reporting(E_ALL); 
echo &#39;<B>未定义$var</b><Br>&#39;; 
echo "isset测试:<Br>"; 
if ( isset ( $var )) 
{ 
echo &#39;变量$var存在!<Br>&#39; ; 
} 
echo "empty测试:<Br>"; 
if ( empty ( $var )){ 
echo &#39;变量$var的值为空<Br>&#39;; 
} 
else 
{ 
echo &#39;变量$var的值不为空<Br>&#39;; 
} 
echo "变量直接测试:<Br>"; 
if ( $var ){ 
echo &#39;变量$var存在!<Br>&#39;; 
} 
else { 
echo &#39;变量$var不存在!<Br>&#39;; 
} 
echo &#39;----------------------------------<br>&#39;; 
echo &#39;<B>$var = \&#39;\&#39;</b><Br>&#39;; 
echo "isset测试:<Br>"; 
$var = &#39;&#39;; 
if ( isset ( $var )) 
{ 
echo &#39;变量$var存在!<Br>&#39; ; 
} 
echo "empty测试:<Br>"; 
if ( empty ( $var )){ 
echo &#39;变量$var的值为空<Br>&#39;; 
} 
else 
{ 
echo &#39;变量$var的值不为空<Br>&#39;; 
} 
echo "变量直接测试:<Br>"; 
if ( $var ){ 
echo &#39;变量$var存在!<Br>&#39;; 
} 
else { 
echo &#39;变量$var不存在!<Br>&#39;; 
} 
echo &#39;----------------------------------<br>&#39;; 
echo &#39;<B>$var = 0</b><Br>&#39;; 
echo &#39;isset测试:<Br>&#39;; 
$var = 0 ; 
if ( isset ( $var )) 
{ 
echo &#39;变量$var存在!<Br>&#39; ; 
} 
echo "empty测试:<Br>"; 
if ( empty ( $var )){ 
echo &#39;变量$var的值为空<Br>&#39;; 
} 
else 
{ 
echo &#39;变量$var的值不为空<Br>&#39;; 
} 
echo "变量直接测试:<Br>"; 
if ( $var ){ 
echo &#39;变量$var存在!<Br>&#39;; 
} 
else { 
echo &#39;变量$var不存在!<Br>&#39;; 
} 
echo &#39;----------------------------------<br>&#39;; 
echo &#39;<B>$var = null</b><Br>&#39;; 
echo &#39;isset测试:<Br>&#39;; 
$var = null ; 
if ( isset ( $var )) 
{ 
echo &#39;变量$var存在!<Br>&#39; ; 
} 
echo "empty测试:<Br>"; 
if ( empty ( $var )){ 
echo &#39;变量$var的值为空<Br>&#39;; 
} 
else 
{ 
echo &#39;变量$var的值不为空<Br>&#39;; 
} 
echo "变量直接测试:<Br>"; 
if ( $var ){ 
echo &#39;变量$var存在!<Br>&#39;; 
} 
else { 
echo &#39;变量$var不存在!<Br>&#39;; 
} 
echo &#39;----------------------------------<br>&#39;; 

echo &#39;<B>$var ="php"</b><Br>&#39;; 
echo &#39;isset测试:<Br>&#39;; 
$var = "php"; 
if ( isset ( $var )) 
{ 
echo &#39;变量$var存在!<Br>&#39; ; 
} 

echo "empty测试:<Br>"; 
if ( empty ( $var )){ 
echo &#39;变量$var的值为空<Br>&#39;; 
} 
else 
{ 
echo &#39;变量$var的值不为空<Br>&#39;; 
} 
echo "变量直接测试:<Br>"; 
if ( $var ){ 
echo &#39;变量$var存在!<Br>&#39;; 
} 
else { 
echo &#39;变量$var不存在!<Br>&#39;; 
} 
?>

When writing page programs in PHP, I often use variable processing functions to determine whether a variable value at the end of the PHP page is empty. At first, I was used to using the empty() function, but I found some problems, so I switched to isset. () function, the problem is no longer there.
As the name suggests, empty() determines whether a variable is "empty", and isset() determines whether a variable has been set. It is this so-called "as the name implies" that made me take some detours at the beginning: when a variable value is equal to 0, empty() will also be true (True), so some accidents will occur. It turns out that although empty() and isset() are both variable processing functions, they are both used to determine whether the variable has been configured, but they have certain differences: empty will also detect whether the variable is empty or zero. When a variable value is 0, empty() considers the variable to be equivalent to being empty, which is equivalent to not being set.
For example, to detect the $id variable, when $id=0, use empty() and isset() to detect whether the variable $id has been configured. Both will return different values ​​- empty( ) that there is no configuration, isset() can get the value of $id:

$id=0; 
empty($id)?print "It's empty .":print "It's $id ."; 
//结果：It's empty . 
print "<br>"; 
!isset($id)?print "It&#39;s empty .":print "It&#39;s $id ."; 
//结果：It&#39;s 0 .

This means that when we use the variable processing function, when the variable may have a value of 0, use Be careful with empty(), it would be wiser to replace it with isset in this case.
When the URL tail parameter of a php page appears id=0 (for example: test.php?id=0), try to compare:

if(empty($id)) $id=1; - 若 id=0 ，id 也会为1 
if(!isset($id)) $id=1; - 若 id=0 ，id 不会为1

The following code can be run separately to detect the above inference:

if(empty($id)) $id=1; 
print $id; // 得到 1 
if(!isset($id)) $id=1; 
print $id; //得到 0

To talk about their connection, their common point is that empty() and isset() are both variable processing functions, and their function is to determine whether the variable is Already configured, it is precisely because of their great similarity in the process of processing variables that they have insufficient understanding of their relationship. Just considering the two functions empty() and isset() themselves will make people more confused. Look at it from another angle. The processing objects of empty() and isset() are nothing more than undefined variables, 0, and empty strings.
If the variable is 0, empty() will return TRUE and isset() will return TRUE;

If the variable is an empty string, empty() will return TRUE, isset() will return TRUE;
If the variable is undefined, empty() will return TRUE, isset() will return FLASE;

The explanation of empty() in the manual is as follows:

Description 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 properties will be considered empty, and TRUE will be returned if var is empty.
The explanation of isset() in the manual is as follows:

isset() detects 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 ("?") is not equivalent to PHP's NULL constant.
Warning: isset() can only be used with variables, because passing any other parameters will cause a parsing error. If you want to check whether a constant has been set, you can use the defined() function.

You can use the isset function when you want to determine whether a variable has been declared.
You can use it when you want to determine whether a variable has been assigned data and is not empty. empty function
When you want to determine that a variable exists and is not empty, first use the isset function and then use the empty function

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PHP The difference between isset() function and empty() function_PHP tutorial

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