When writing PHP applications, you may need to compute a value difference using a custom comparison callback. This is useful in everyday tasks such as working with imported data, application settings, user-facing lists, and reports.
The array_udiff() operation provides this behavior directly. The examples below show what happens to the inputs and what to check before using the result in your application.
What Is array_udiff() in PHP?
Compute a value difference using a custom comparison callback.
Its syntax is:
array_udiff(array $array, array ...$arrays, callable $value_compare_func): array
Parameters used by this operation:
$array— The collection being examined or updated.$arrays— The collection being examined or updated.$value_compare_func— A callable that controls the comparison or transformation; callbacks should follow the expected return contract.
The callback must return negative, zero or positive integers, not a boolean; keys from the first array are retained.
Example 1: Basic Usage
Start with a small, self-contained input so you can inspect exactly how the operation behaves.
<?php
$first = ["ALPHA", "beta", "gamma"];
$second = ["alpha", "DELTA"];
$result = array_udiff($first, $second, 'strcasecmp');
print_r($result);
Output:
Array
(
[1] => beta
[2] => gamma
)
The output shows the result of the first call. Check the printed keys and values as well as their order.
Example 2: Another Important Case
This variation highlights another input shape, a comparison option, or a change in the collection.
<?php
$inventory = [7, 15, 23];
$excluded = [15];
print_r(array_udiff($inventory, $excluded, fn($left, $right) => $left <=> $right));
Output:
Array
(
[0] => 7
[2] => 23
)
Compare this result with Example 1: the difference comes from the input or option selected, not from additional processing.
Practical Example: Exclude a list of banned labels using a case-insensitive comparator
Exclude a list of banned labels using a case-insensitive comparator. Save the code in a PHP file and adapt the sample data to your project.
<?php
$catalogLabels = ["Standard", "Premium", "Enterprise"];
$blockedLabels = ["premium"];
$allowedLabels = array_udiff($catalogLabels, $blockedLabels, "strcasecmp");
echo implode(", ", $allowedLabels);
Output:
Standard, Enterprise
The output comes from the completed workflow shown above. When adapting it, change the source records and keep the same checks for missing, unexpected, or invalid values.
Important Behavior and Edge Cases
The callback must return negative, zero or positive integers, not a boolean; keys from the first array are retained.
- Test the operation with empty or missing input and decide how your application should respond.
- Read the return value independently from any changes to variables passed by reference.
- Validate untrusted input before using the result for permissions, database changes, or user-visible output.
Comparison With Related Functions
| Function or Approach | When to Use It |
|---|---|
array_udiff() | Compute a value difference using a custom comparison callback |
| Related functions | array_diff() compares values without a custom callback; array_udiff_assoc() also checks keys. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Returning true or false from a comparator | Return a negative integer, zero, or a positive integer to express ordering. |
| Mixing up key and value callbacks | Pay attention to the comparator order and which part of each entry it compares. |
| Assuming input values are rewritten | The callback must return negative, zero or positive integers, not a boolean; keys from the first array are retained. |
Key Takeaways
array_udiff()helps you compute a value difference using a custom comparison callback.- The callback must return negative, zero or positive integers, not a boolean; keys from the first array are retained.
- array_diff() compares values without a custom callback; array_udiff_assoc() also checks keys.
- Use the practical example as a starting point, but validate real application inputs.
array_udiff() is useful when you need to compute a value difference using a custom comparison callback. Use its documented rules and the examples above to choose the right behavior for your data.