Detailed explanation of the difference between PHP isset and empty

PHP’s isset() function is generally used to detect whether a variable is set
Format: bool isset (mixed var [, mixed var [, ...]])
Function: Detect whether a variable is set
Return value:
If the variable does not exist Then return FALSE
If the variable exists and its value is NULL, it also returns FALSE
If the variable exists and its value is not NULL, it returns TRUE
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 For "", 0, "0", NULL,, FALSE, array(), var $var; and objects without any attributes, TRUE is returned
If the variable exists and the value is not "", 0, "0", NULL, FALSE, array(), var $var; and objects without any attributes will return FALSE
Versions: PHP 3, PHP 4, PHP 5
More instructions:
empty() return value =!(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()

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

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 tested code of a detailed example of isset and empty functions. After reading this, it is basically the same:

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

</span><span>echo</span> '<b>$var ="php"</b><br>'<span>; 
</span><span>echo</span> 'isset测试:<br>'<span>; 
</span><span>$var</span> = "php"<span>; 
</span><span>if</span> ( <span>isset</span> ( <span>$var</span><span> )) 
{ 
</span><span>echo</span> '变量$var存在!<br>'<span> ; 
} 

</span><span>echo</span> "empty测试:<br>"<span>; 
</span><span>if</span> ( <span>empty</span> ( <span>$var</span><span> )){ 
</span><span>echo</span> '变量$var的值为空<br>'<span>; 
} 
</span><span>else</span><span>
{ 
</span><span>echo</span> '变量$var的值不为空<br>'<span>; 
} 
</span><span>echo</span> "变量直接测试:<br>"<span>; 
</span><span>if</span> ( <span>$var</span><span> ){ 
</span><span>echo</span> '变量$var存在!<br>'<span>; 
} 
</span><span>else</span><span> { 
</span><span>echo</span> '变量$var不存在!<br>'<span>; 
} 
</span>?>

When using PHP to write page programs, I often use variable processing functions to determine the PHP page tail parameters Whether the value of a certain variable is empty. At the beginning, I was used to using the empty() function, but found some problems, so I switched to the isset() function and the problem no longer existed.
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() considers it not configured, isset() Able to get the value of $id:

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

This means that when we use the variable processing function, when the variable may have a value of 0, be careful when using empty(). It is more sensible to replace it with isset at this time.
When the URL tail parameter of a php page appears id=0 (for example: test.php?id=0), try to compare:

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

You can run the following code separately to detect the above inference:

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

To talk about their connection, their common point is that empty() and isset() are both variable processing functions. Their 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 leading to insufficient understanding of their relationship. If you only consider the two functions empty() and isset() themselves, it 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, 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, Then 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, as 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 judge whether a variable has been declared, you can use the isset function
When you want to judge whether a variable has been assigned data and is not empty, you can use the empty function
When you want to judge whether a variable exists and is not empty, first use the isset function and then Use empty function

The above introduces the detailed explanation of the difference between PHP isset and empty, including the relevant aspects. I hope it will be helpful to friends who are interested in PHP tutorials.