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.

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

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
?>

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'.

"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.

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.

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.

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.

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'
?>

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).

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.

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.

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 );
?>

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

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 that array keys are case sensitive.

<?php
$ar['w'] = true;

var_dump(isset($ar['w']),
      isset($ar['W']));
?>

will report:
bool(true) bool(false)

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.