The difference and choice between is_a() and instanceof

M66 2025-06-01

1. Syntax and usage

1. is_a() function

 is_a(object|string $object_or_class, string $class, bool $allow_string = false): bool

$object_or_class : can be an object or class name string.
$class : The target class name to be judged.
$allow_string : If the passed in is a class name string instead of an object, this parameter must be set to true .

Example:

 class Animal {}
class Dog extends Animal {}

$dog = new Dog();

var_dump(is_a($dog, 'Animal')); // true

You can also pass in strings:

 var_dump(is_a('Dog', 'Animal', true)); // true

2. instanceof operator

 $object instanceof ClassName

Example:

 $dog = new Dog();
var_dump($dog instanceof Animal); // true

Note: instanceof can only be used for objects, and cannot be used directly for class name strings.

2. Return result comparison

Both return a Boolean value to determine whether the object is an instance or subclass of a certain class. but:

is_a() is more relaxed and can judge the class name string (in the case of $allow_string = true ).
instanceof is more strict and more object-oriented, and cannot judge class name strings.

3. Performance differences

In most scenarios, the performance gap between the two can be ignored. But from the perspective of the language, instanceof is an operator that acts directly on the object and has slightly better performance. And is_a() is a function with certain calling overhead.

Unless you perform type judgments in a very frequent loop, you don't need to pay too much attention to this.

IV. Compatibility and Risk

is_a() was a similar internal function before PHP 5, and it officially supported the current form since PHP 5.
instanceof is a language operator and has been stable in support.

If you need to make your code compatible with earlier versions or avoid function calls (such as function overrides, mocks, etc.), instanceof is more reliable.

5. Safety considerations

When making judgments using the class name provided by the user, is_a() is easily misused. For example:

 $class = $_GET['class']; // Users may pass in any class name
if (is_a($object, $class)) {
    // ...
}

This type of dynamic judgment has certain safety risks. Using instanceof can better constrain type structures and avoid potential injections or misjudgments.

6. Practical suggestions

It is recommended to use instanceof first, for the following reasons:

More semantic and in line with the object-oriented programming style.
Less error risk, clearer type logic.
Slightly excellent performance and simpler expression.

When a few people need to pass in class names dynamically (such as factory mode, plug-in mechanism), is_a() can be used, but explicitly set $allow_string = true and ensure that the passed in class names are safe and trustworthy.

For example:

 $class = 'App\\Plugins\\SomeHandler';
if (is_a($handler, $class, true)) {
    $handler->handle();
}

Reference dynamic plugin management sample code hosted in:

 https://m66.net/examples/plugin-loader.php

is_a