The difference between isset() and empty() in PHP

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, it will also return FALSE
If the variable exists and If the value is not NULL, TRUE will be returned
When checking multiple variables at the same time, TRUE will be returned only when each item meets the previous requirement, otherwise the result will be FALSE
Version: 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 none If the object has any attribute, it will return FALSE
Versions: PHP 3, PHP 4, PHP 5
More instructions:
The return value of empty() =!(boolean) var, but it will not be because the variable is not Define a warning message. 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 between 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 be called by variable functions.
Note: empty() only detects variables, 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 using PHP to write page programs, I often use variables The processing function determines 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 the isset() function and the problem no longer existed.
As the name suggests, empty() Determine whether a variable is "empty", 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 it will happen Some surprises. It turns out that empty() and isset() Although they 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() This variable is considered to be equal to empty, which is equivalent to no setting.
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() considers 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, be careful when using empty() and replace it with isset at this time. Be smarter.
When the URL tail parameter of a php page appears id=0 (for example: test.php?id=0), try to compare:

if(empty($id)) $id=1; - If id=0, id will also be 1
if(!isset($id)) $id=1; - If id=0, id will not be 1

You can run the following code separately Check the above inference:

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

To talk about their connection, their common points are empty() and isset() is a variable processing function, and its function is to determine whether the variable has been configured. It is precisely because of their great similarity in the process of processing variables that they have insufficient understanding of their relationship. Shan Cong If the two functions empty() and isset() are considered by themselves, it will make people more confused. Look at them 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, isset() will return TRUE;

If the variable is an empty string, empty() will return TRUE, isset() will return TRUE ;
If the variable is not defined, empty() will return TRUE and 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.

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

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