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array_flip() + array_diff_key() implements differential operation

M66 2025-05-18

In PHP, there are many ways to process array difference sets. In addition to the common array_diff() function, flexible difference set calculation can be achieved through the "key" method with the help of array_flip() and array_diff_key() functions. This method is particularly efficient and practical when dealing with the "key-value matching differences" between two arrays.

1. Review of basic concepts

1. array_flip()

array_flip() will interchange the keys and values ​​of the array. This means that the original value will become a key, and the key will become a value.

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

// Output：['red' => 'apple', 'yellow' => 'banana']

Note: If there are duplicate values ​​in the original array, array_flip() will discard the duplicates and only the last one is retained.

2. array_diff_key()

array_diff_key() is used to compare keys of two or more arrays and return key-value pairs that exist in the first array but do not appear in other arrays.

 $array1 = ['a' => 1, 'b' => 2, 'c' => 3];
$array2 = ['b' => 99];

$result = array_diff_key($array1, $array2);

// Output：['a' => 1, 'c' => 3]

2. Use array_flip + array_diff_key to implement "value difference set"

Suppose we have two arrays and want to find the unique value (i.e., the difference set) in the first array, we can cleverly use the combination of array_flip() and array_diff_key() to achieve it.

Sample code:

 $groupA = ['apple', 'banana', 'cherry'];
$groupB = ['banana', 'grape'];

// Convert value to key
$flippedA = array_flip($groupA);
$flippedB = array_flip($groupB);

// Differential operation（pass key Difference）
$diff = array_diff_key($flippedA, $flippedB);

// Restored to a normal value array
$uniqueToA = array_keys($diff);

print_r($uniqueToA);

Output result:

 Array
(
    [0] => apple
    [1] => cherry
)

We successfully found the value that is in $groupA but not in $groupB .

3. Examples of application scenarios

This method is very suitable for the following scenarios:

  • Quickly find out the difference between two label arrays;

  • User permission filtering (find out features that the user has permission but the module is not used);

  • Form multiple options comparison differences, etc.

For example, you are developing a user permission system, and the current user permissions are:

 $userPermissions = ['edit', 'delete', 'create'];

The operations supported by a certain module are:

 $moduleSupports = ['edit', 'create'];

You can find out the permissions that the user has but the module does not support in the same way:

 $diff = array_keys(array_diff_key(
    array_flip($userPermissions),
    array_flip($moduleSupports)
));

// Output ['delete']

4. Summary

Although PHP provides a difference set that is directly used for evaluation, in some more refined scenarios (such as when the value is unique), the combination of array_flip ( ) and array_diff_key() is not only efficient, but also provides more flexible operation methods.

This type of technique can bring higher performance especially when dealing with large arrays or when comparing operators are required.

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