PHP isset() Function

The PHP isset() function checks whether a variable is declared and is different than null. A variable is considered set, if the variable is declared and it is different than null.

If multiple parameters are supplied then this function returns true only if all of the parameters are considered set. It evaluate variables from left to right and stops as soon as an unset variable is found.

If a variable has been unset with the unset() function, it is no longer considered to be set.

This function returns false when checking a variable that has been assigned to null. Note that a null character ("\0") is not equivalent to the PHP null constant.

Syntax

isset(var, vars)

Parameters

Return Value

Returns true if var exists and has any value other than null. false otherwise.

Example: isset() examples

The example below shows the usage of isset() function.

<?php
$a = null;
$b = '';
$x = 'test';
$y = 10;

var_dump(isset($a));         //returns: bool(false)
var_dump(isset($x, $a));     //returns: bool(false)

echo "\n";

var_dump(isset($b));          //returns: bool(true)
var_dump(isset($x, $y));      //returns: bool(true)
var_dump(isset($b, $x, $y));  //returns: bool(true)

echo "\n";

//unset variable $x
unset($x);

var_dump(isset($x, $y));      //returns: bool(false)
var_dump(isset($b, $x, $y));  //returns: bool(false)

echo "\n";
//this will evaluate to TRUE so the text will be printed
if (isset($y)) {
  echo "y is set to $y";
}
?>

The output of the above code will be:

bool(false)
bool(false)

bool(true)
bool(true)
bool(true)

bool(false)
bool(false)

y is set to 10

Example: using isset() on arrays

The isset() function also works in the similar way on elements of an array. Consider the example below:

<?php
$Arr1 = array(10, null);

$Arr2 = array("a" => 10,
              "b" => null); 

//checking elements of $Arr1
var_dump(isset($Arr1[0]));
var_dump(isset($Arr1[1]));
var_dump(isset($Arr1[10]));

echo "\n";

//checking elements of $Arr2
var_dump(isset($Arr2['a']));
var_dump(isset($Arr2['b']));
var_dump(isset($Arr2['z']));
?>

The output of the above code will be:

bool(true)
bool(false)
bool(false)

bool(true)
bool(false)
bool(false)