(PHP 4, PHP 5, PHP 7, PHP 8)
isset — 检测变量是否已设置并且非 null
检测变量是否设置，并且不是 null。
如果已经使用 unset()
释放了一个变量之后，它将不再是
isset()。若使用 isset()
测试一个被设置成 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() 只有在全部参数都以被设置时返回 true 计算过程从左至右，中途遇到没有设置的变量时就会立即停止。
var要检查的变量。
...其他变量。
如果 var 存在并且值不是 null 则返回 true，否则返回 false。
| 版本 | 说明 |
|---|---|
| 5.4.0 |
检查字符的非数字偏移量将会返回 |
Example #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
?>
Example #2 在字符串位移中使用 isset()
PHP 5.4 改变了传入字符串位移时 isset() 的行为。
<?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']));
?>
以上例程在 PHP 5.3 中的输出：
bool(true) bool(true) bool(true) bool(true) bool(true) bool(true)
以上例程在 PHP 5.4 中的输出：
bool(false) bool(true) bool(true) bool(true) bool(false) bool(false)
Note: 因为是一个语言构造器而不是一个函数，不能被 可变函数 调用。
Note:
如果使用 isset() 来检查对象无法访问的属性，如果 __isset() 方法已经定义则会调用这个重载方法。
In these days you can use ISSET to compare a sting length
COOOL! I love php !!!!
<?php
//FIRST:try to run the script like this,
//THEN: try to uncomment the seccond $str=
$yes="SURE!!!";
$no="NOPE !";
$str="12345";//the string is long 5 bytes ?
//$str="123456";
echo !isset($str{5}) ? $yes : $no;
?>
Would be nice to have is_set() alias for more language consistency (alike of is_int, is_bull so on).
Be warned that before checking an array key existence with isset() the key will be typecasted to integer if it is not a string or integer!
See Example #2 in http://php.net/manual/en/language.types.array.php
I.e.
$a = ['test']; // 'test' will have key 0
$key = false;
var_dump(isset($a[$key]) ); // true, because false will be typecasted to 0
It is not a problem with isset but rather the lose typing when you access keys in arrays
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).
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'] ?? $_POST['user'] ?? 'nobody';
?>
Quoted from http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op
I, too, was dismayed to find that isset($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.
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.
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( $var );
?>
Note that isset() is not recursive as of the 5.4.8 I have available here to test with: if you use it on a multidimensional array or an object it will not check isset() on each dimension as it goes.
Imagine you have a class with a normal __isset and a __get that fatals for non-existant properties. isset($object->nosuch) will behave normally but isset($object->nosuch->foo) will crash. Rather harsh IMO but still possible.
<?php
class FatalOnGet {
// pretend that the methods have implementations that actually try to do work
// in this example I only care about the worst case conditions
public function __get($name) {
echo "(getting {$name}) ";
// if property does not exist {
echo "Property does not exist!";
exit;
// }
}
public function __isset($name) {
echo "(isset {$name}?) ";
// return whether the property exists
return false;
}
}
$obj = new FatalOnGet();
// works
echo "Testing if ->nosuch exists: ";
if (isset($obj->nosuch)) echo "Yes"; else echo "No";
// fatals
echo "\nTesting if ->nosuch->foo exists: ";
if (isset($obj->nosuch->foo)) echo "Yes"; else echo "No";
// not executed
echo "\nTesting if ->irrelevant exists: ";
if (isset($obj->irrelevant)) echo "Yes"; else echo "No";
?>
Testing if ->nosuch exists: No
Testing if ->nosuch->foo exists: Property does not exist!
Uncomment the echos in the methods and you'll see exactly what happened:
Testing if ->nosuch exists: (isset nosuch?) No
Testing if ->nosuch->foo exists: (getting nosuch) Property does not exist!
On a similar note, if __get always returns but instead issues warnings or notices then those will surface.
How to test for a variable actually existing, including being set to null. This will prevent errors when passing to functions.
<?php
// false
var_export(
array_key_exists('myvar', get_defined_vars())
);
$myvar;
// false
var_export(
array_key_exists('myvar', get_defined_vars())
);
$myvar = null;
// true
var_export(
array_key_exists('myvar', get_defined_vars())
);
unset($myvar);
// false
var_export(
array_key_exists('myvar', get_defined_vars())
);
if (array_key_exists('myvar', get_defined_vars())) {
myfunction($myvar);
}
?>
Note: you can't turn this into a function (e.g. is_defined($myvar)) because get_defined_vars() only gets the variables in the current scope and entering a function changes the scope.
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'
?>
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
?>
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.
Note that array keys are case sensitive.
<?php
$ar['w'] = true;
var_dump(isset($ar['w']),
isset($ar['W']));
?>
will report:
bool(true) bool(false)
You can safely use isset to check properties and subproperties of objects directly. So instead of writing
isset($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
?>
I know this is probably not the recommended way to do this, but it seems to work fine for me. Instead of the normal isset check to extract variables from arrays (like $_REQUEST), you can use the @ prefix to squelch any errors.
For example, instead of:
<?php
$test = isset($_REQUEST['test']) ? $_REQUEST['test'] : null;
?>
you can use:
<?php
$test = @$_REQUEST['test'];
?>
It saves some typing, but doesn't give the opportunity to provide a default value. If 'test' is not an assigned key for $_REQUEST, the assigned value will be null.
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.
isset expects the variable sign first, so you can't add parentheses or anything.
<?php
$foo = 1;
if(isset(($foo))) { // Syntax error at isset((
$foo = 2;
}
?>
To organize some of the frequently used functions..
<?php
/**
* Returns field of variable (arr[key] or obj->prop), otherwise the third parameter
* @param array/object $arr_or_obj
* @param string $key_or_prop
* @param mixed $else
*/
function nz($arr_or_obj, $key_or_prop, $else){
$result = $else;
if(isset($arr_or_obj)){
if(is_array($arr_or_obj){
if(isset($arr_or_obj[$key_or_prop]))
$result = $arr_or_obj[$key_or_prop];
}elseif(is_object($arr_or_object))
if(isset($arr_or_obj->$key_or_prop))
$result = $arr_or_obj->$key_or_prop;
}
}
return $result;
}
/**
* Returns integer value using nz()
*/
function nz_int($arr_or_obj, $key_or_prop, $else){
return intval(nz($arr_or_obj, $key_or_prop, $else));
}
$my_id = nz_int($_REQUEST, 'id', 0);
if($my_id > 0){
//why?
}
?>
Note: isset() only checks variables as anything else will result in a parse error. In other words, the following will not work: isset(trim($name)).
isset() is the opposite of is_null($var) , except that no warning is generated when the variable is not set.
In PHP 5.2.3, really returns true if the variable is set to null.
"empty() is the opposite of (boolean) var, except that no warning is generated when the variable is not set."
So essentially
<?php
if (isset($var) && $var)
?>
is the same as
<?php
if (!empty($var))
?>
doesn't it? :)
!empty() mimics the chk() function posted before.
The unexpected results of isset has been really frustrating to me. Hence, it doesn't work how you'd think it would, (as documented) a var currently in the scope with a null value will return false.
Heres a quick solution, perhaps there are better ways of going about this, but heres my solution...
<?php
function is_set( $varname, $parent=null ) {
if ( !is_array( $parent ) && !is_object($parent) ) {
$parent = $GLOBALS;
}
return array_key_exists( $varname, $parent );
}
?>
Hence, $varname should be a mixed value of var's to check for, and $parent can be an array or object, which will default to the GLOBAL scope. See the documentation of array_key_exists for further information.
This will allow to check if a var is in the current scope, object, or array... Whether it's a null, false, true, or any value. It depends on ARRAY_KEY_EXISTS for it's functionality which also works with Objects. Feel free to improve on this anyone ;D
In response to 10-Feb-2006 06:02, isset($v) is in all (except possibly buggy) cases equivalent to !is_null($v). And no, it doesn't actually test if a variable is set or not by my definition "$v is set if unset($v) has no effect".
<?php
unset($c); //force $c to be unset
var_dump($a=&$c); // NULL, but this actually sets $a and $c to the 'same' NULL.
var_dump(isset($c)); // bool(false)
var_dump($a = 5); // int(5)
var_dump($c); // int(5)
unset($c);
var_dump($a=&$c); // NULL
var_dump(isset($c)); // bool(false)
unset($c);
var_dump($a = 5); // int(5)
var_dump($c); // NULL
?>
In the following example, we see an alternate method of testing if a variable is actually set or not:
<?php
var_dump(array_key_exists('c',get_defined_vars())); // false
var_dump(isset($c)); // also false
var_dump($c); // manipulate $c a bit...
var_dump((string)$c);
var_dump(print_r($c,true));
var_dump($a=$c);
var_dump(array_key_exists('c',get_defined_vars())); // ... still false
var_dump($c = NULL); // this sets $c
var_dump(array_key_exists('c',get_defined_vars())); // true!
var_dump(isset($c)); // false; isset() still says it's unset
unset($c); // actually unset it
var_dump(array_key_exists('c',get_defined_vars())); // false
var_dump($a=&$c);
var_dump(array_key_exists('c',get_defined_vars())); // true!
unset($c); // unset it again
var_dump(&$c); // &NULL
var_dump(array_key_exists('c',get_defined_vars())); // true!
?>
Obviously, null values take up space (or they wouldn't show up in get_defined_vars). Also, note that &$v sets $v to NULL if it is unset.
I tried the example posted previously by Slawek:
$foo = 'a little string';
echo isset($foo)?'yes ':'no ', isset($foo['aaaa'])?'yes ':'no ';
He got yes yes, but he didn't say what version of PHP he was using.
I tried this on PHP 5.0.5 and got: yes no
But on PHP 4.3.5 I got: yes yes
Apparently, PHP4 converts the the string 'aaaa' to zero and then returns the string character at that position within the string $foo, when $foo is not an array. That means you can't assume you are dealing with an array, even if you used an expression such as isset($foo['aaaa']['bbb']['cc']['d']), because it will return true also if any part is a string.
PHP5 does not do this. If $foo is a string, the index must actually be numeric (e.g. $foo[0]) for it to return the indexed character.
Sometimes you have to check if an array has some keys. To achieve it you can use "isset" like this: isset($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.
<?php
$foo = 'a little string';
echo isset($foo)?'yes ':'no ', isset($foo['aaaa'])?'yes ':'no ';
>
results with unexpected values:
yes yes
Well, it is necessary to check type of $foo first !
The following is an example of how to test if a variable is set, whether or not it is NULL. It makes use of the fact that an unset variable will throw an E_NOTICE error, but one initialized as NULL will not.
<?php
function var_exists($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.
in PHP5, 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'.
This function is very useful while calling to the URL to specify which template to be used on certain parts of your application.
Here is an example...
<?php
$cat = $_GET['c'];
$id = $_GET['id'];
$error = 'templates/error.tpl';
if( isset($cat))
{
if( isset($id))
{
$var = 'templates/pics/' . $cat . '-' . $id . '.tpl';
if (is_file($var))
{
include($var);
}
else
{
include($error);
}
}
else
{
$var = 'templates/pics/' . $cat . '.tpl';
if (is_file($var))
{
include($var);
}
else
{
include($error);
}
}
}
else
{
include('templates/alternative.'.tpl);
}
?>
You can see several uses of the isset function being used to specify wheter a template is to be called upon or not. This can easily prevent other generic PHP errors.