Why can't array_flip() be used to process object arrays?

M66 2025-05-17

In PHP, array_flip() is a very convenient function that can be used to put arrays. It sounds very simple and practical, but you have to be careful when using it, especially when an array contains objects. This article will explore why arrays containing objects cannot be processed with array_flip() and what problems will be encountered when doing so.

1. The basic principle of array_flip()

 $input = [
    'apple' => 'red',
    'banana' => 'yellow',
];
$flipped = array_flip($input);
print_r($flipped);

Output:

 Array
(
    [red] => apple
    [yellow] => banana
)

As shown above, key-value pairs are swapped, no problem. But please note one key point: the flipped key must be a legal array key , that is, the int or string type.

2. What happens if an array contains objects?

Let's take a look at an example containing objects:

 class User {
    public $name;
    function __construct($name) {
        $this->name = $name;
    }
}

$user1 = new User('Alice');
$user2 = new User('Bob');

$array = [
    'u1' => $user1,
    'u2' => $user2,
];

$flipped = array_flip($array);

When running this code, PHP will throw a warning or even a fatal error:

 Warning: array_flip(): Can only flip STRING and INTEGER values!

Why? Because objects cannot be used as keys to arrays. The key types of arrays in PHP are limited by integers and strings. Objects (including anonymous classes) are not legal key types.

3. Further example: Mixed arrays

 $array = [
    'a' => 'apple',
    'b' => new stdClass(),
    'c' => 'carrot'
];

$flipped = array_flip($array);

Output:

 Warning: array_flip(): Can only flip STRING and INTEGER values!

Even if only one value is an object, the entire array_flip() operation will fail. This will cause you to lose all your data, or program interruptions.

4. What is the correct way to do it?

If you do need to flip an array containing objects, you usually need to handle this process manually. For example, you can manually build an inverted array based on a certain property of the object:

 $array = [
    'u1' => new User('Alice'),
    'u2' => new User('Bob'),
];

$flipped = [];

foreach ($array as $key => $user) {
    if ($user instanceof User) {
        $flipped[$user->name] = $key;
    }
}

print_r($flipped);

Output:

 Array
(
    [Alice] => u1
    [Bob] => u2
)

This way you can implement the "flip" function based on a unique property of the object, instead of using array_flip() directly.

5. Summary

array_flip() is a powerful tool, but it is not omnipotent. When an array you process contains objects or other data types that cannot be used as keys (such as arrays, resources, etc.), it throws an error or even interrupts the script. To ensure the robustness of the code, you should always confirm the type of the value before calling, or use a more fault-tolerant alternative.

array_flip