说明
判断一个变量是否已设置,
即变量已被声明，且其值不为 null。
如果一个变量已经被使用 unset() 释放，它将不再被认为已设置。
若使用 isset()
测试一个被赋值为 null 的变量，将返回 false。
同时要注意的是 null 字符（"\0"）并不等同于
PHP 的 null 常量。
如果一次传入多个参数，那么
isset() 只有在全部参数都已被设置时返回 true。 计算过程从左至右，中途遇到未设置的变量时就会立即停止。
参数
var-
要检查的变量。
vars-
其他变量。
示例
示例 #1 isset() 例子
<?php
$var = '';
// 结果为 TRUE，所以后边的文本将被打印出来。
if (isset($var)) {
echo "This var is set so I will print.";
}
// 在后边的例子中，我们将使用 var_dump 输出 isset() 的返回值。
// the return value of isset().
$a = "test";
$b = "anothertest";
var_dump(isset($a)); // TRUE
var_dump(isset($a, $b)); // TRUE
unset ($a);
var_dump(isset($a)); // FALSE
var_dump(isset($a, $b)); // FALSE
$foo = NULL;
var_dump(isset($foo)); // FALSE
?>这对于数组中的元素也同样有效：
<?php
$a = array ('test' => 1, 'hello' => NULL, 'pie' => array('a' => 'apple'));
var_dump(isset($a['test'])); // TRUE
var_dump(isset($a['foo'])); // FALSE
var_dump(isset($a['hello'])); // FALSE
// 键 'hello' 的值等于 NULL，所以被认为是未置值的。
// 如果想检测 NULL 键值，可以试试下边的方法。
var_dump(array_key_exists('hello', $a)); // TRUE
// Checking deeper array values
var_dump(isset($a['pie']['a'])); // TRUE
var_dump(isset($a['pie']['b'])); // FALSE
var_dump(isset($a['cake']['a']['b'])); // FALSE
?>示例 #2 在字符串位移中使用 isset()
<?php
$expected_array_got_string = 'somestring';
var_dump(isset($expected_array_got_string['some_key']));
var_dump(isset($expected_array_got_string[0]));
var_dump(isset($expected_array_got_string['0']));
var_dump(isset($expected_array_got_string[0.5]));
var_dump(isset($expected_array_got_string['0.5']));
var_dump(isset($expected_array_got_string['0 Mostel']));
?>以上示例会输出：
bool(false) bool(true) bool(true) bool(true) bool(false) bool(false)
参见
- empty() - 检查变量是否为空
- __isset()
- unset() - 清除指定变量
- defined() - 检查给定名称的常量是否存在
- the type comparison tables
- array_key_exists() - 检查数组里是否有指定的键名或索引
- is_null() - 检测变量是否是 null
- 错误控制 @ 运算符。
＋添加备注
用户贡献的备注 17 notes
p_ignorethis_lbowers at gmail dot com ¶
8 years ago
I, too, was dismayed to find that isset($foo) returns false if ($foo == null). Here's an (awkward) way around it.
unset($foo);
if (compact('foo') != array()) {
do_your_thing();
}
Of course, that is very non-intuitive, long, hard-to-understand, and kludgy. Better to design your code so you don't depend on the difference between an unset variable and a variable with the value null. But "better" only because PHP has made this weird development choice.
In my thinking this was a mistake in the development of PHP. The name ("isset") should describe the function and not have the desciption be "is set AND is not null". If it was done properly a programmer could very easily do (isset($var) || is_null($var)) if they wanted to check for this!
A variable set to null is a different state than a variable not set - there should be some easy way to differentiate. Just my (pointless) $0.02.
kurdtpage at gmail dot com ¶
8 years ago
The new (as of PHP7) 'null coalesce operator' allows shorthand isset. You can use it like so:
<?php
// Fetches the value of $_GET['user'] and returns 'nobody'
// if it does not exist.
$username = $_GET['user'] ?? 'nobody';
// This is equivalent to:
$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';
// Coalescing can be chained: this will return the first
// defined value out of $_GET['user'], $_POST['user'], and
// 'nobody'.
$username = $_GET['user'] ?? $_POST['user'] ?? 'nobody';
?>
Quoted from http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op
a dot schaffhirt at sedna-soft dot de ¶
16 years ago
You can safely use isset to check properties and subproperties of objects directly. So instead of writing
isset($abc) && isset($abc->def) && isset($abc->def->ghi)
or in a shorter form
isset($abc, $abc->def, $abc->def->ghi)
you can just write
isset ($abc->def->ghi)
without raising any errors, warnings or notices.
Examples
<?php
$abc = (object) array("def" => 123);
var_dump(isset($abc)); // bool(true)
var_dump(isset($abc->def)); // bool(true)
var_dump(isset($abc->def->ghi)); // bool(false)
var_dump(isset($abc->def->ghi->jkl)); // bool(false)
var_dump(isset($def)); // bool(false)
var_dump(isset($def->ghi)); // bool(false)
var_dump(isset($def->ghi->jkl)); // bool(false)
var_dump($abc); // object(stdClass)#1 (1) { ["def"] => int(123) }
var_dump($abc->def); // int(123)
var_dump($abc->def->ghi); // null / E_NOTICE: Trying to get property of non-object
var_dump($abc->def->ghi->jkl); // null / E_NOTICE: Trying to get property of non-object
var_dump($def); // null / E_NOTICE: Trying to get property of non-object
var_dump($def->ghi); // null / E_NOTICE: Trying to get property of non-object
var_dump($def->ghi->jkl); // null / E_NOTICE: Trying to get property of non-object
?>
yaogzhan at gmail dot com ¶
19 years ago
If you have
<?PHP
class Foo
{
protected $data = array('bar' => null);
function __get($p)
{
if( isset($this->data[$p]) ) return $this->data[$p];
}
}
?>
and
<?PHP
$foo = new Foo;
echo isset($foo->bar);
?>
will always echo 'false'. because the isset() accepts VARIABLES as it parameters, but in this case, $foo->bar is NOT a VARIABLE. it is a VALUE returned from the __get() method of the class Foo. thus the isset($foo->bar) expreesion will always equal 'false'.
eckoson at gmail dot com ¶
1 year ago
If you are annoyed by the behavior of isset() concerning null values, here is a handy function for you. its similar to array_key_exists but, its a lot more flexible and can check for multiple array keys across multiple arrays.
Not recursive!
Tested on php 8.1.6, linux
<?php
/**
* is_set
* @author DJ Eckoson
* @link @eckosongh Facebook Page
* checks whether variable names are set within the global space or they exists as an key and return if they are set (even if their values are null)
* @param string $var_name
* name of the first variable to check
* @param array|null|string
* optional array to check for key (if null, checks from $GLOBALS) OR
* other variable names to check OR
* other variable names and their associated arrays to their right (use null for global variables, optional if its the last parameter)
* check examples below
*/
function is_set(string $var_name, array|null|string ... $args): bool {
$vars[$var_name] = null;
if (array_key_exists(0, $args)) {
if (is_array($args[0])) {
$vars[$var_name] = $args[0];
} elseif (is_string($args[0])) {
goto main;
}
unset($args[0]);
}
main:
if ($args) {
$args = array_reverse($args);
$cur_array = null;
array_walk($args, function ($value) use (&$cur_array, &$vars): void {
if (!is_string($value)) {
$cur_array = $value;
} else {
$vars[$value] = $cur_array;
}
});
}
foreach ($vars as $name => $array) {
if (!array_key_exists($name, $array??$GLOBALS)) return false;
}
return true;
}
// Examples
$arr1 = range(0, 5);
$arr2 = [
'a' => 1,
'b' => 2,
'c' => 'hELLO wORLD!'
];
$gender = 'male';
$age = 12;
var_dump(is_set('age')); // true
var_dump(is_set('age', null)); // true
var_dump(is_set('age', $arr1)); // false
var_dump(is_set('age', array())); // false
var_dump(is_set('age', array('age' => 48))); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender')); // true
var_dump(is_set('age', 'arr1', null, 'b', $arr2, 0, 3, 4, $arr1, 'gender', null)); // true
$c=$d=$e=$a=2;
$arr = [1,4];
var_dump(is_set('a', 'c', null, 0, 1, $arr)); // true
var_dump(is_set('a', 'c', null, 0, 4, $arr)); // false
?>
Note:
it won't work for variables declared locally inside a function;
however you can use it for checking if array keys exists inside a function
beuc at beuc dot net ¶
18 years ago
"empty() is the opposite of (boolean) var, except that no warning is generated when the variable is not set."
So essentially
<?php
if (isset($var) && $var)
?>
is the same as
<?php
if (!empty($var))
?>
doesn't it? :)
!empty() mimics the chk() function posted before.
ayyappan dot ashok at gmail dot com ¶
8 years ago
Return Values :
Returns TRUE if var exists and has value other than NULL, FALSE otherwise.
<?php
$a=NULL;
$b=FALSE; //The out put was TRUE.
$c=TRUE;
$d='';
$e="";
if(isset($b)):
echo "TRUE";
else:
echo "FALSE";
endif;
?>
Could any one explain me in clarity.
soywiz at php dot net ¶
18 years ago
Sometimes you have to check if an array has some keys. To achieve it you can use "isset" like this: isset($array['key1'], $array['key2'], $array['key3'], $array['key4'])
You have to write $array all times and it is reiterative if you use same array each time.
With this simple function you can check if an array has some keys:
<?php
function isset_array() {
if (func_num_args() < 2) return true;
$args = func_get_args();
$array = array_shift($args);
if (!is_array($array)) return false;
foreach ($args as $n) if (!isset($array[$n])) return false;
return true;
}
?>
Use: isset_array($array, 'key1', 'key2', 'key3', 'key4')
First parameter has the array; following parameters has the keys you want to check.
mandos78 AT mail from google ¶
16 years ago
Careful with this function "ifsetfor" by soapergem, passing by reference means that if, like the example $_GET['id'], the argument is an array index, it will be created in the original array (with a null value), thus causing posible trouble with the following code. At least in PHP 5.
For example:
<?php
$a = array();
print_r($a);
ifsetor($a["unsetindex"], 'default');
print_r($a);
?>
will print
Array
(
)
Array
(
[unsetindex] =>
)
Any foreach or similar will be different before and after the call.
Hayley Watson ¶
7 years ago
If you regard isset() as indicating whether the given variable has a value or not, and recall that NULL is intended to indicate that a value is _absent_ (as said, somewhat awkwardly, on its manual page), then its behaviour is not at all inconsistent or confusing.
It's not just to check for uninitialised variables - a lot of the time those are just due to sloppy coding. There are other ways a variable could fail to have a value (e.g., it's meant to hold the value returned from a function call but the function didn't have a value to return) where uninitialising the variable would not be an option nor even make sense (e.g., depending on what was to be done with the returned value).
Cuong Huy To ¶
13 years ago
1) Note that isset($var) doesn't distinguish the two cases when $var is undefined, or is null. Evidence is in the following code.
<?php
unset($undefined);
$null = null;
if (true === isset($undefined)){echo 'isset($undefined) === true'} else {echo 'isset($undefined) === false'); // 'isset($undefined) === false'
if (true === isset($null)){echo 'isset($null) === true'} else {echo 'isset($null) === false'); // 'isset($null) === false'
?>
2) If you want to distinguish undefined variable with a defined variable with a null value, then use array_key_exist
<?php
unset($undefined);
$null = null;
if (true !== array_key_exists('undefined', get_defined_vars())) {echo '$undefined does not exist';} else {echo '$undefined exists';} // '$undefined does not exist'
if (true === array_key_exists('null', get_defined_vars())) {echo '$null exists';} else {echo '$null does not exist';} // '$null exists'
?>
packard_bell_nec at hotmail dot com ¶
17 years ago
Note: isset() only checks variables as anything else will result in a parse error. In other words, the following will not work: isset(trim($name)).
isset() is the opposite of is_null($var) , except that no warning is generated when the variable is not set.
Ashus ¶
16 years ago
Note that array keys are case sensitive.
<?php
$ar['w'] = true;
var_dump(isset($ar['w']),
isset($ar['W']));
?>
will report:
bool(true) bool(false)
Andrew Penry ¶
19 years ago
The following is an example of how to test if a variable is set, whether or not it is NULL. It makes use of the fact that an unset variable will throw an E_NOTICE error, but one initialized as NULL will not.
<?php
function var_exists($var){
if (empty($GLOBALS['var_exists_err'])) {
return true;
} else {
unset($GLOBALS['var_exists_err']);
return false;
}
}
function var_existsHandler($errno, $errstr, $errfile, $errline) {
$GLOBALS['var_exists_err'] = true;
}
$l = NULL;
set_error_handler("var_existsHandler", E_NOTICE);
echo (var_exists($l)) ? "True " : "False ";
echo (var_exists($k)) ? "True " : "False ";
restore_error_handler();
?>
Outputs:
True False
The problem is, the set_error_handler and restore_error_handler calls can not be inside the function, which means you need 2 extra lines of code every time you are testing. And if you have any E_NOTICE errors caused by other code between the set_error_handler and restore_error_handler they will not be dealt with properly. One solution:
<?php
function var_exists($var){
if (empty($GLOBALS['var_exists_err'])) {
return true;
} else {
unset($GLOBALS['var_exists_err']);
return false;
}
}
function var_existsHandler($errno, $errstr, $errfile, $errline) {
$filearr = file($errfile);
if (strpos($filearr[$errline-1], 'var_exists') !== false) {
$GLOBALS['var_exists_err'] = true;
return true;
} else {
return false;
}
}
$l = NULL;
set_error_handler("var_existsHandler", E_NOTICE);
echo (var_exists($l)) ? "True " : "False ";
echo (var_exists($k)) ? "True " : "False ";
is_null($j);
restore_error_handler();
?>
Outputs:
True False
Notice: Undefined variable: j in filename.php on line 26
This will make the handler only handle var_exists, but it adds a lot of overhead. Everytime an E_NOTICE error happens, the file it originated from will be loaded into an array.
andreasonny83 at gmail dot com ¶
9 years ago
Here is an example with multiple parameters supplied
<?php
$var = array();
$var['val1'] = 'test';
$var['val2'] = 'on';
if ( isset( $var['val1'], $var['val2'] ) && $var['val2'] === 'on' ) {
unset( $var['val1'] );
}
print_r( $var );
?>
This will output:
Array
(
[val2] => on
)
The following code does the same calling "isset" 2 times:
<?php
$var = array();
$var['val1'] = 'test';
$var['val2'] = 'on';
if ( isset( $var['val1'] ) && isset( $var['val2'] ) && $var['val2'] === 'on' ) {
unset( $var['val1'] );
}
print_r( $var );
?>
francois vespa ¶
14 years ago
Now this is how to achieve the same effect (ie, having isset() returning true even if variable has been set to null) for objects and arrays
<?php
// array
$array=array('foo'=>null);
return isset($array['foo']) || array_key_exists('foo',$array)
? true : false ; // return true
return isset($array['inexistent']) || array_key_exists('inexistent',$array)
? true : false ; // return false
// static class
class bar
{
static $foo=null;
}
return isset(bar::$foo) || array_key_exists('foo',get_class_vars('bar'))
? true : false ; // return true
return isset(bar::$inexistent) || array_key_exists('inexistent',get_class_vars('bar'))
? true : false ; // return false
// object
class bar
{
public $foo=null;
}
$bar=new bar();
return isset($bar->foo) || array_key_exists('foo',get_object_vars($bar))
? true : false ; // return true
return isset($bar->inexistent) || array_key_exists('inexistent',get_object_vars($bar))
? true : false ; // return true
// stdClass
$bar=new stdClass;
$bar->foo=null;
return isset($bar->foo) || array_key_exists('foo',get_object_vars($bar))
? true : false ; // return true
return isset($bar->inexistent) || array_key_exists('inexistent',get_object_vars($bar))
? true : false ; // return true
?>
Anl zselgin ¶
15 years ago
Note: Because this is a language construct and not a function, it cannot be called using variable functions.
So why it is under "Variable handling Functions". Maybe there should be some good documentation field for language constructs.