PHP isset(): Check if a Variable or Array Key Is Set

isset($var) returns true if the variable exists and its value is not null. It returns false for an undefined variable, an unset one, or one holding null, and it never prints a warning.

<?php
$name = "Ada";
$age = 0;
$nickname = null;

var_dump(isset($name));       // exists, not null
var_dump(isset($age));        // 0 is still set
var_dump(isset($nickname));   // null counts as not set
var_dump(isset($undefined));  // never assigned

unset($name);
var_dump(isset($name));       // removed with unset()

isset() only looks at whether something is there. A value that is set but empty, like 0, "" or [], still returns true. If you want "set and not empty", use empty (negated).

Check if an array key exists with isset

This is where isset() is used most. Reading a key that does not exist prints Warning: Undefined array key on a normal PHP setup; checking it with isset() first does not. It also works for nested keys, and stops quietly at the first missing level.

<?php
$user = [
    "name" => "Grace",
    "email" => "grace@example.com",
    "address" => ["city" => "Arlington"],
    "phone" => null,
];

var_dump(isset($user["email"]));
var_dump(isset($user["website"]));
var_dump(isset($user["address"]["city"]));
var_dump(isset($user["address"]["zip"]));
var_dump(isset($user["company"]["name"]));  // parent key missing too
var_dump(isset($user["phone"]));            // key exists, value is null

if (isset($user["email"])) {
    echo "Send mail to {$user['email']}\n";
}

When you only need the value or a default, $user["website"] ?? "none" does the same check in one step. See the null coalescing operator.

isset with multiple variables

Pass several arguments and isset() returns true only if all of them are set and not null. It is a short way to check that every required field arrived.

<?php
$data = ["first" => "Ada", "last" => "Lovelace", "email" => null];

var_dump(isset($data["first"], $data["last"]));
var_dump(isset($data["first"], $data["email"]));

$required = ["first", "last", "email"];
$missing = array_filter($required, fn($key) => !isset($data[$key]));
echo "Missing: " . implode(", ", $missing) . "\n";

isset on strings and objects

On a string, isset($str[5]) checks whether there is a character at offset 5, which is a quick length check. On an object, it checks a property; for a class with private data, isset() calls the __isset() method if the class defines one.

<?php
$code = "AB12";
var_dump(isset($code[3]));   // 4th character exists
var_dump(isset($code[4]));   // there is no 5th

class Profile
{
    public ?string $bio = null;
    public string $name = "Linus";
    private array $extra = ["twitter" => "@linus"];

    public function __isset(string $key): bool
    {
        return isset($this->extra[$key]);
    }
}

$p = new Profile();
var_dump(isset($p->name));
var_dump(isset($p->bio));       // null
var_dump(isset($p->twitter));   // answered by __isset()
var_dump(isset($p->github));

isset vs empty vs is_null

Pick a value of $x in the table to compare the three checks and the plain (bool) cast. Every cell was computed by PHP 8.3.

The pattern to remember:

• isset() is false only for null and things that do not exist.
• empty() is true for those and also for every falsy value: false, 0, 0.0, "", "0", [].
• is_null() gives the opposite of isset() for existing variables, but warns when the variable is undefined. Use isset() or === null instead.

Check if a form was submitted

isset() is the classic way to see whether a form field arrived. Run this block, type a name and press Send: the same script runs again with $_POST filled in.

<?php
$message = "";
if (isset($_POST["name"])) {
    $name = trim($_POST["name"]);
    $message = $name === ""
        ? "<p style='color:#c33'>Please type a name.</p>"
        : "<p>Hello, " . htmlspecialchars($name) . "</p>";
}
?>
<form method="post">
  <label>Name <input name="name"></label>
  <label><input type="checkbox" name="news" value="1"> Newsletter</label>
  <button>Send</button>
</form>
<?= $message ?>
<?php if (isset($_POST["name"])): ?>
  <p>Newsletter: <?= isset($_POST["news"]) ? "yes" : "no" ?></p>
<?php endif; ?>

An empty text field is still sent, as "", so isset($_POST["name"]) is true after any submit. An unchecked checkbox is not sent at all, which makes isset() exactly the right test for checkboxes. To require a non-empty value, check the trimmed string as the block does. The forms page builds a full form with validation.

isset vs array_key_exists

The two agree on everything except a key whose value is null. isset() treats it as missing, array_key_exists() does not.

<?php
$settings = ["timeout" => 30, "proxy" => null];

foreach (["timeout", "proxy", "retries"] as $key) {
    printf("%-8s isset: %-5s  array_key_exists: %s\n",
        $key,
        var_export(isset($settings[$key]), true),
        var_export(array_key_exists($key, $settings), true));
}

If "set to null" means something different from "not configured" in your data, use array_key_exists(). Otherwise isset() is shorter, and it is a language construct rather than a function call, so it is also slightly faster.

Common mistake: isset on an expression

isset() only accepts variables, array elements and properties. Passing a function result or a constant is a compile error:

<?php
if (isset(trim($input))) {}

PHP Fatal error:  Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in /home/index.php on line 2

Store the result in a variable first, or test the value itself: null !== $expr as the message suggests, or trim($input) !== "" for a string.