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What is array_intersect_ukey() in PHP? Full Guide with Examples (2025)

📖 Introduction to PHP Array Functions

PHP offers a rich set of built-in array functions that make working with arrays fast, reliable, and convenient. Among these, the array_intersect_ukey() function holds a unique spot because it compares the keys of multiple arrays using a custom callback function for comparison, and returns the matching key-value pairs from the first array.

This tutorial will guide you through:

  • What array_intersect_ukey() is

  • Syntax and parameters

  • Practical real-world examples

  • SEO-friendly FAQs

  • Differences with related functions

  • Performance considerations

  • Best practices for clean, secure, and efficient PHP coding


📌 What is array_intersect_ukey() in PHP?

The array_intersect_ukey() function compares the keys of two or more arrays using a user-defined callback function. It returns an array containing all the values of the first array whose keys are present in all the other arrays, as determined by the callback.

It’s particularly useful when you need fine control over how array keys are compared — for example, case-insensitive key comparisons, or numeric comparisons with specific logic.


📌 Syntax of array_intersect_ukey()


array_intersect_ukey(array $array1, array $array2, ..., callable $key_compare_func): array
  • $array1, $array2, ... — Arrays to compare.

  • $key_compare_func — A custom callback function to compare the keys.

Returns:
An array containing entries from $array1 whose keys are present in all the other arrays.


📌 Parameters Explanation

ParameterDescription
$array1The array to compare from.
$array2, ...Arrays to compare against.
$key_compare_funcUser-defined function that takes two keys and returns 0, 1, or -1.



The callback should follow:
function compare($a, $b) { if ($a === $b) return 0; return ($a > $b) ? 1 : -1; }

📌 Basic Example of array_intersect_ukey()


function compareKeys($key1, $key2) { return strcmp($key1, $key2); } $array1 = ["a" => "Apple", "b" => "Banana", "c" => "Cherry"]; $array2 = ["a" => "Apricot", "c" => "Citrus", "d" => "Date"]; $result = array_intersect_ukey($array1, $array2, "compareKeys"); print_r($result);

Output:


Array ( [a] => Apple [c] => Cherry )

📌 Real-World Use Cases of array_intersect_ukey()

✅ Case-Insensitive Key Intersection


function compareCaseInsensitive($key1, $key2) { return strcasecmp($key1, $key2); } $array1 = ["A" => 1, "B" => 2, "C" => 3]; $array2 = ["a" => 4, "c" => 6]; $result = array_intersect_ukey($array1, $array2, "compareCaseInsensitive"); print_r($result);

✅ Comparing Numeric Keys with Custom Rules


function numericCompare($a, $b) { return ($a == $b) ? 0 : ($a > $b ? 1 : -1); } $array1 = [100 => "Data1", 200 => "Data2"]; $array2 = [100 => "AnotherData", 300 => "Else"]; $result = array_intersect_ukey($array1, $array2, "numericCompare"); print_r($result);

📌 How array_intersect_ukey() Works Internally

  1. PHP iterates through $array1.

  2. For each key in $array1, it compares it against keys in the other arrays using the callback.

  3. If the callback returns 0 (indicating equality) for any key in each of the other arrays, that key-value pair from $array1 is included in the result.

  4. It repeats until all keys are processed.


📌 Difference Between array_intersect_ukey() and Similar Functions

FunctionWhat it ComparesComparison Type
array_intersect()Array valuesUsing ==
array_intersect_key()Array keysUsing ==
array_intersect_assoc()Both keys and valuesUsing ==
array_intersect_ukey()Array keysUsing custom callback



📌 Performance Considerations
  • For small arrays, performance differences are negligible.

  • For large arrays, array_intersect_ukey() may perform slower than native array_intersect_key() due to the overhead of executing the callback.

  • Custom callback functions that are computationally expensive (e.g., string operations) can significantly affect speed.

Optimization Tips:

  • Use native functions when possible.

  • Keep callback functions lightweight.

  • Avoid expensive operations inside callbacks when working with large arrays.


📌 Common Errors and How to Avoid Them

1️⃣ Missing callback function


// Will throw a warning array_intersect_ukey($array1, $array2);

2️⃣ Passing wrong data types


// Passing a non-array will cause a warning array_intersect_ukey($array1, "not_an_array", "callbackFunc");

3️⃣ Callback returns non-integer


function badCompare($a, $b) { return true; // should be 0, 1 or -1 }

Fix: Ensure your callback returns 0, 1, or -1.


📌 Nested Arrays with array_intersect_ukey()

It doesn’t check nested arrays by default. Only the top-level keys are compared.

Example:


$array1 = ["a" => ["x" => 1], "b" => 2]; $array2 = ["a" => 3]; $result = array_intersect_ukey($array1, $array2, "strcmp"); print_r($result);

Output:


Array ( [a] => Array ( [x] => 1 ) )

📌 Best Practices

  • Always validate input arrays before using them in this function.

  • Keep callback logic simple to optimize performance.

  • Document your callback functions clearly to avoid confusion.

  • Avoid side effects in your callback function.

  • Use array_intersect_key() when a simple key comparison without custom logic suffices.


📌 SEO-Friendly FAQs

Q: What does array_intersect_ukey() do in PHP?
A: It compares keys of multiple arrays using a custom callback function and returns matching key-value pairs from the first array.

Q: How is array_intersect_ukey() different from array_intersect_key()?
A: array_intersect_key() uses a simple equality check ==, while array_intersect_ukey() uses a user-defined comparison function.

Q: Can array_intersect_ukey() compare values too?
A: No, it compares only keys. Use array_intersect_assoc() or array_intersect() for value comparisons.

Q: Is array_intersect_ukey() case-sensitive?
A: By default, yes — unless you provide a case-insensitive callback like strcasecmp().

Q: Can I use anonymous functions as the callback?
A: Yes, PHP 5.3+ supports anonymous functions.


$result = array_intersect_ukey($array1, $array2, function($a, $b) { return strcmp($a, $b); });

📌 Summary

The array_intersect_ukey() function is an advanced yet highly useful array comparison tool in PHP. Its flexibility lies in allowing custom comparison logic for keys, making it ideal for specialized applications like case-insensitive, numeric, or format-specific key comparisons.

Key Takeaways:

  • Use it when you need custom key comparison logic.

  • Always ensure your callback returns 0, 1, or -1.

  • Avoid using it for large arrays with expensive callbacks.

  • Use native alternatives (array_intersect_key(), array_intersect()) for simpler use-cases.


📌 Final Thought

Knowing when and how to use array_intersect_ukey() can help PHP developers write cleaner, efficient, and more maintainable code. It may not be used every day, but when the situation arises — you’ll be ready.


📌 Meta Title (SEO Example)

PHP array_intersect_ukey() Function Guide | Syntax, Examples & Use Cases (2025)

📌 Meta Description (SEO Example)

Learn how to use PHP's array_intersect_ukey() function to compare array keys with a custom callback. Explore syntax, practical examples, performance tips, and SEO FAQs.


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