usort vs uasort: Sorting PHP Associative Arrays

When sorting arrays in PHP using custom comparison logic, developers often choose between the usort() and uasort() functions. While both functions allow you to define your own sorting criteria using a callback function, they behave very differently regarding how they handle array keys. This article explains the fundamental differences between usort() and uasort(), demonstrating how each function affects associative arrays so you can choose the correct tool for your PHP projects.

The Core Difference

The main distinction between these two functions lies in key preservation:

• usort() sorts the array by value using a user-defined comparison function and discards the original keys, assigning new numerical keys starting from 0.
• uasort() sorts the array by value using a user-defined comparison function and maintains the original key-value associations.

How usort() Works

Use usort() when the original keys of your array do not matter, such as in simple list arrays (indexed arrays). If you pass an associative array to usort(), all your custom string or non-sequential keys will be lost and replaced with sequential integers.

Example of usort()

$fruits = [
    'apple'  => 3,
    'banana' => 1,
    'cherry' => 2
];

// Sort ascending by value
usort($fruits, function($a, $b) {
    return $a <=> $b;
});

print_r($fruits);

Output:

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)

Note that the original keys (apple, banana, cherry) have been completely destroyed and replaced with 0, 1, and 2.

How uasort() Works

Use uasort() when sorting associative arrays where the keys contain essential relationship data (such as IDs, usernames, or configuration names) that must remain linked to their respective values.

Example of uasort()

$fruits = [
    'apple'  => 3,
    'banana' => 1,
    'cherry' => 2
];

// Sort ascending by value while maintaining keys
uasort($fruits, function($a, $b) {
    return $a <=> $b;
});

print_r($fruits);

Output:

Array
(
    [banana] => 1
    [cherry] => 2
    [apple] => 3
)

The array is successfully sorted by the numeric values, but the original associative keys remain securely mapped to their values.

Summary: When to Use Which?