PHP isset

Summary: in this tutorial, you will learn how to use the PHP isset() construct to check if a variable is set and not null.

Introduction to the PHP isset() construct #

PHP isset() returns true if a variable is set and not null.

isset(mixed $var): bool

The isset() is a language construct, not a function. Therefore, you cannot assign it to a variable, return it from a function, or call it dynamically via a variable function.

The following example will result in an error:

<?php

$f = isset;

Error:

PHP Parse error:  syntax error, unexpected ';', expecting '('

To work around it, you can create a function that uses the isset() construct and call that function using the variable functions. For example:

<?php

function isset_and_not_null($var): bool
{
    return isset($var);
}

Or it’s shorter if you use the arrow function syntax:

$isset = fn($var) => isset($var);

var_dump($isset($count)) // false

Note that you’ll learn about the arrow function syntax in the later tutorial.

The following returns false because the $count variable has not been declared:

<?php

var_dump(isset($count));

Try it

Output:

bool(false)

The following example returns true because the $count variable is declared, and its value is different than null:

<?php

$count = 0;
var_dump(isset($count));

Try it

Output:

bool(true)

If you assign null to a variable and pass it to the isset(), the isset() will return false:

<?php

$count = null;
var_dump(isset($count));

Try it

Output:

bool(false)

If you unset a variable, the variable becomes unset. Therefore, the isset() will return false:

<?php

$count = 0;
unset($count);
var_dump(isset($count));

Try it

Output:

bool(false)

Using PHP isset with an array #

If you pass an array element to isset(), it’ll return true. For example:

<?php

$colors = ['primary' => 'blue'];

var_dump(isset($colors['primary']));

Try it

Output:

bool(true)

However, if you pass a non-existing element to isset(), it’ll return false. For example:

<?php

$colors = ['primary' => 'blue'];

var_dump(isset($colors['secondary']));

Try it

Output:

bool(true)

If you pass an array element with value null to isset(), The isset() will return false:

<?php

$colors = ['primary' => 'blue','secondary' => null];

var_dump(isset($colors['secondary']));

Try it

Output:

bool(false)

PHP isset() with string offsets #

The isset() works with string offsets. For example:

<?php

$message = 'Hello';
var_dump(isset($message[0]));

Try it

Output:

bool(true)

If you pass a string element with an invalid offset, the isset() will return false. For example:

<?php

$message = 'Hello';
var_dump(isset($message[strlen($message)]));

Try it

Output:

bool(false)

PHP isset() with multiple variables #

The isset() accepts multiple variables and returns true if all variables are set. The isset() It evaluates the variables from left to right and stops when encountering an unset variable.

isset(mixed $v1, mixed $v2, ...): bool

The following example returns true because all variables $x, $y and $z are set:

<?php

$x = 10;
$y = 20;
$z = 30;

var_dump(isset($x, $y, $z));

Try it

Output:

bool(true)

However, the following example returns false because the variable $y is null, which is not set. Also, the isset() won’t evaluate the variable $z:

<?php

$x = 10;
$y = null;
$z = 30;

var_dump(isset($x, $y, $z));

Try it

Output:

bool(false)

Summary #

isset() is a language construct, not a function.
isset() returns true if a variable is set and not null.
isset() returns true if an array element exists and is not null.
isset() returns true if a string index is valid or false otherwise.
isset() returns true if all variables are set and not null. It’ll stop evaluating once it encounters an unset variable.

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