isset() is one of the most important tools at your disposal to validate data in PHP. Like the name implies, it is designed to verify if a variable given to it is set, returning a boolean value based on the result.

However, it has some quirks and behaviours that are very much worth knowing as they can easily catch out even experienced developers.

Let's take a look through how it behaves and what's so special about it. Even if you're a veteran PHP developer, hopefully, you'll pick up something new here.

It's a language construct, NOT a function

Despite looking like one, isset() is actually NOT a typical function as you might expect.

Just like die(), array(), print() and others, it is a "language construct", which is a fancy term that, in laymen's terms, means it's built directly into the PHP engine and can have some special behaviour that is different than typical built-in or user-defined functions, which we will go over next.

It can not be used as a callable

Any built-in or user-defined function can be used as a "callable" function pointer to be invoked dynamically and used for patterns like currying.

is_callable('strtoupper');
// true

array_map('strtoupper', ['a', 'b', null, 'd']);
// ['A', 'B', '', 'D']

As it is a language construct and not really a function, it is not callable and cannot be used in such a way.

is_callable('isset');
// false

array_map('isset', ['a', 'b', null, 'd']);
// PHP Warning:  array_map() expects parameter 1 to be a valid callback, function 'isset' not found or invalid function name…

It does not accept an expression

While regular functions and other language constructs can accept the result of any expression, due to its unique nature, isset() can only accept a variable, array key or object property as an argument.

Attempting to use it any other way will result in a fatal error.

if (isset('Hello world')) {
    // Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
}

if (isset($a->b())) {
    // Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
}

if (isset(! $a)) {
    // Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
}

if (isset(CONSTANT)) {
    // Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
}

It also checks if a value is null

isset() will return false if a variable is undefined OR if its value is null. This may throw you off if null is a proper value you have set and want to allow.

$value = null;

if (isset($value)) {
    // ...
}

One way to go about checking this, depending on your requirements, is to check if a variable is defined in the current scope with get_defined_vars() and examining the resulting array's keys.

$value = null;

if (array_key_exists('value', get_defined_vars())) {
    // ...
}

It can accept multiple arguments

It's very common to see people chaining calls together to check that multiple values are set one-by-one.

if (isset($a) && isset($b) && isset($c)) {
    // ...
}

However, isset() is a variadic function that can accept any number of parameters at once to achieve the same effect, confirming if all of the passed variables are set.

This can be a great way to shorten long conditionals.

if (isset($a, $b, $c)) {
    // ...
}

It does not trigger "undefined variable/index/property" notices

If you're retrieving a value nested multiple levels deep, you probably want to make sure every step of the chain exists.

if (isset($response, $response->list, $response->list['results'], $response->list['results'][0])) {
    // ...
}

if (isset($arr[$key], $otherArr[$arr[$key]], $otherArr[$arr[$key]][$otherKey])) {
    // ...
}

However, isset() will not trigger any "undefined variable", "undefined index" or "undefined property" notices, no matter how many layers you go through.

This means that instead of confirming the value at every individual step, they can all be done in a single check:

if (isset($response->list['results'][0])) {
    // ...
}

if (isset($otherArr[$arr[$key]][$otherKey])) {
    // ...
}

"Undefined method" errors do get triggered

If a chain being checked happens to include a method call halfway through it, PHP will attempt to invoke the method.

This means that if an earlier part of the chain does not exist, or the last value in the chain is an object that simply does not have this method, an error will still be triggered.

$a = new stdClass();

if (isset($a->b()->c)) {
    // Fatal error: Uncaught Error: Call to undefined method A::b()…
}

if (isset($a->b->c())) {
    // Fatal error: Uncaught Error: Call to a member function d() on null…
}

One way to deal with this is to be explicit in your conditional checks, stopping the chain and calling method_exists() to verify the method exists every time it is needed.

if (isset($a) && method_exists($a, 'b') && isset($a->b()->c)) {
    // ...
}

One way to shorten such an expression is to use the error control operator, which suppresses any errors for a single expression. If an error is triggered, the operator will make the expression return null instead and continue the execution.

if (@$a->b()->c !== null) {
    // ...
}

However, while this may be convenient, you should be aware that the error control operator is very inefficient and can also suppress errors triggered within the called methods you call and are not intending to suppress. It is not an outright replacement for isset().

!empty() is not quite the same

[empty()](https://www.php.net/manual/en/function.empty.php) is also a language construct with similar behaviour to isset() in that it doesn’t trigger undefined notices.

$a = [];

if (empty($a['b']->c)) {
   // ...
}

It seems as if it serves as a direct inverse of isset(), but this is not the case. empty() can also accept expressions as its arguments, but more importantly, it will type juggle so that any falsey value is treated as such.

$a = '0';

if (isset($a)) {
    // It IS set
}

if (empty($a)) {
    // It IS empty
}

Null coalesce operator

It is a very common occurrence to want to provide a fallback value in case a variable is not set. This is typically done with a short conditional if statement or ternary clause.

$result = isset($value) ? $value : 'fallback';

As of PHP 7.0, this can be shortened using the null coalesce operator (??) which will return the first value if it is set, or the second value if not.

$result = $value ?? 'fallback';

If instead of returning a new value, you didn't want to set a new variable doing this, that is covered as well. As of PHP 7.4, the null coalesce assignment operator (??=) allows an even shorter way to set a variable to a fallback if it isn't already set.

$value ??= 'fallback';

It does not evaluate the __get() magic method

Let's assume we have a pretty typical class that can dynamically get properties by using the magic method __get() to retrieve a value.

class Person
{
    protected $attributes = [];

    public function __get($name)
    {
        return $this->attributes[$name] ?? null;
    }

    public function __set($name, $value)
    {
        $this->attributes[$name] = $value;
    }
}

If we use this class to set a property, we can make use of it as we might normally expect. However, if we check if the value is set, it will return false

$person = new Person();
$person->name = 'Liam';

echo $person->name; // 'Liam'

isset($person->name); // false

Wait, what's going on here?!

Because isset() is a language construct and not a regular function, the expression doesn't get evaluated before it's passed to it. Because name isn't a real property on the object, it doesn't really exist.

However, when isset() gets called on a property that doesn't exist or is inaccessible to the current scope (such as being protected or private), it will invoke a magic __isset() method if the class has one defined. This allows for custom logic to be done to determine if we think the property we're checking is set according to our own rules.

class Person
{
    // ...

    public function __isset($name)
    {
        return isset($this->attributes[$name]);
    }
}

With that implemented, everything works as expected.

$person = new Person();
$person->name = 'Liam';

isset($person->name); // true
isset($person->somethingElse); // false

It is important to note that if you are checking nested properties, __isset() will be invoked as appropriate for each property in the chain.

You can pass non-existent variables to userland functions

As we have already discussed, because isset() is actually a language construct, it has special behaviour because of the PHP core, and thus does not behave like functions we define ourselves.

However, we can achieve a similar effect in userland functions, through the use of references. By doing this, we open up the possibility to expose additional functionality of our own choosing on top of the regular language construct.

One practical example of this might be to treat any objects implementing a null object pattern as falsey values.

interface NullObject {}

class Logger {
    // ...
}

class NullLogger extends Logger implements NullObject {
    // ...
}

function is_truthy(&$value)
{
    if ($value instanceof NullObject) {
        return false;
    }

    return (bool) $value;
}

is_truthy($a);
// false

$b = '';
is_truthy($b);
// false

$c = '1';
is_truthy($c);
// true

$logger = new Logger();
is_truthy($logger);
// true

$nullLogger = new NullLogger();
is_truthy($nullLogger);
// false

However, references are not always that safe to use, as simply using them can affect the original value, even if the function doesn't explicitly do it.

For example, any undefined array keys or properties will automatically be assigned and their value set to null

$a = [];

is_truthy($a['b']['c']);

// false

var_dump($a);

// [

//     'b' => [

//         'c' => null,

//     ],

// ]

Conclusion

Hopefully throughout this look at isset(), its behaviour and other related things, you will have picked something up that will help you make your code cleaner, more explicit, and not catch you out in edge cases when you need to check if a variable has been set.