Detailed explanation of the difference between isset() and empty() in php

Through learning the PHP language, you should know that it is a function-based HTML scripting language. A huge function library supports the implementation of PHP language functions. Below we will introduce to you the related usage of PHP functions 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, it returns 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, each single item matches the previous one Returns TRUE only when requested, 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 parsing errors.
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, it returns TRUE
If If the variable exists and its value is "", 0, "0", NULL, FALSE, array(), var $var; and an object without any attributes, TRUE is returned. If the variable exists and its 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:

empty() The 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()

<?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 code for a detailed example of isset and empty functions that we have tested. 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 variable processing functions to determine certain parameters at the end of the PHP page. Whether the value of each 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 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(). 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:

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

You can run the following code 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 say Their connection has in common 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 are Insufficient understanding of relationships. 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 Definition, 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, then 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 return 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, use the isset function first and then use it. empty function