php empty(), isset(), is_null() function usage examples_PHP tutorial

In php, the three functions empty(), isset(), and is_null() are similar to many friends, but if we think about it briefly, they seem to be different. One is a null value, one is a variable, and whether it is null. Below I Let’s give an example to illustrate. ​

Many people are vague about the usage of PHP's empty(), isset() and is_null() functions. There is a lot of information on the Internet, but they may not be able to explain it clearly. Here is a test example, which is helpful for understanding this. The distinction between the three functions can be of great benefit.

The types of tests are as follows:

$a;
$b = false;
$c = '';
$d = 0;
$e = null;
$f = array();
?>

empty()

The code is as follows

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代码如下 复制代码 var_dump(empty($a)); var_dump(empty($b)); var_dump(empty($c)); var_dump(empty($d)); var_dump(empty($e)); var_dump(empty($f)); ?> // 输出 bool(true) bool(true) bool(true) bool(true) bool(true) bool(true)  var_dump(empty($a)); var_dump(empty($b)); var_dump(empty($c)); var_dump(empty($d)); var_dump(empty($e)); var_dump(empty($f)); ?>  代码如下 复制代码 var_dump(isset($a)); var_dump(isset($b)); var_dump(isset($c)); var_dump(isset($d)); var_dump(isset($e)); var_dump(isset($f)); ?> // 输出 bool(false) bool(true) bool(true) bool(true) bool(false) bool(true)  // Output bool(true) bool(true) bool(true) bool(true) bool(true) bool(true)

As can be seen from the code, empty() outputs true as long as the data type is empty or false.

代码如下	复制代码
$id=0;     empty($id)?print "It's empty .":print "It's $id .";       //结果：It's empty .     print " ";     !isset($id)?print "It's empty .":print "It's $id .";       //结果：It's 0 .

isset()

The code is as follows	Copy code
代码如下 复制代码 if(empty($id)) $id=1; - 若 id=0 ，id 也会为1 if(!isset($id)) $id=1; - 若 id=0 ，id 不会为1 var_dump(isset($a)); var_dump(isset($b)); var_dump(isset($c)); var_dump(isset($d)); var_dump(isset($e)); var_dump(isset($f)); ?> // Output bool(false) bool(true) bool(true) bool(true) bool(false) bool(true)

It can be seen that isset() can only be used to determine whether it is NULL and undefined. Warning: isset() can only be used with variables, as passing any other arguments 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 whether a variable exists and is not empty, first use the isset function and then use the empty function 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 () can get the value of $id:

The code is as follows	Copy code
$id=0; empty($id)?print "It's empty .":print "It's $id ."; //Result: It's empty . Print " "; !isset($id)?print "It's empty .":print "It's $id ."; //Result: It's 0 .

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

The code is as follows	Copy code
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

The above inference can be detected by running the following code separately:

The code is as follows	Copy code
代码如下 复制代码  if(empty($id)) $id=1;     print $id; // 得到 1     if(!isset($id)) $id=1; print $id; //得到 0 if(empty($id)) $id=1; Print $id; // get 1 if(!isset($id)) $id=1; print $id; //get 0

代码如下	复制代码
var_dump(is_null($a)); var_dump(is_null($b)); var_dump(is_null($c)); var_dump(is_null($d)); var_dump(is_null($e)); var_dump(is_null($f)); ?> // 输出 bool(true) bool(false) bool(false) bool(false) bool(true) bool(false)

is_null()

The code is as follows	Copy code
var_dump(is_null($a)); var_dump(is_null($b)); var_dump(is_null($c)); var_dump(is_null($d)); var_dump(is_null($e)); var_dump(is_null($f)); ?> // Output bool(true) bool(false) bool(false) bool(false) bool(true) bool(false)

is_null is literal.

It can be seen that empty() can be used to determine whether all data types are empty or false, while is_null is basically the same as isset and can only be used to determine whether it is NULL and undefined.

I will attach a table for you later

The following table can clearly illustrate the relationship between them:

Variable is_null isset
$a=”” true
$a=null                                                                                                                                                   because var $a                                                                                                                                                                                                   because $a=array()                                                                                                                                                                                                                                                               because $a=false                                                    “ $a=15                                                                                                                                                                                                                                                       Since $a=1                                                                                                                                                                                                                                                                                                    because false $a=0                                                                                                                                                                                                         because $a="0" true
$ a = "True" False False True
$ a = "False" False False True





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