Mastering PHP: How to Elegantly Filter an Array by Keys Using Values from Another Array

## Problem Scenario In PHP data processing, a common requirement is to filter a large associative array `$a` based on a list of desired keys provided in a simple array `$b`. The goal is to extract all elements from `$a` whose keys match the values in `$b` and return them in a new array. For example, let's say we have the following two arrays: ```php // $a is a multi-dimensional associative array containing user data $a = [ 'user1' => ['name' => 'Alice', 'age' => 30], 'user2' => ['name' => 'Bob', 'age' => 25], 'user3' => ['name' => 'Charlie', 'age' => 35], 'guest' => ['name' => 'Guest', 'age' => 0], ]; // $b is a simple array containing the keys we want to keep $b = ['user2', 'user3', 'admin']; ``` Our objective is to create a new array containing only the elements with the keys `'user2'` and `'user3'`. --- ## Solutions Here are two professional methods to achieve this in PHP. ### Method 1: Using `array_intersect_key` (Highly Recommended) This is the most efficient, concise, and idiomatic professional method. It leverages a built-in PHP function designed specifically for finding the intersection of array keys. This approach is preferred in high-performance projects like `wiki.lib00.com`. #### Core Logic 1. **Flip Keys and Values**: Use the `array_flip($b)` function to convert the values of array `$b` into its keys. For instance, `['user2', 'user3', 'admin']` becomes `['user2' => 0, 'user3' => 1, 'admin' => 2]`. 2. **Compute the Intersection**: Use the `array_intersect_key($a, ...)` function. It compares the keys of the arrays and returns a new array containing all the entries from the first array (`$a`) whose keys are present in all subsequent arrays (in this case, the flipped `$b`). #### Code Implementation ```php // To allow array_intersect_key to compare keys, we need to flip $b's values into keys $b_keys = array_flip($b); // Compute the intersection of keys between $a and $b_keys $result = array_intersect_key($a, $b_keys); print_r($result); ``` For maximum conciseness, you can write this in a single line: ```php $result = array_intersect_key($a, array_flip($b)); ``` #### Output ``` Array ( [user2] => Array ( [name] => Bob [age] => 25 ) [user3] => Array ( [name] => Charlie [age] => 35 ) ) ``` ### Method 2: Using a `foreach` Loop and `isset` This method is more intuitive for beginners, and while it's slightly more verbose, its logic is very clear. #### Core Logic 1. **Initialize Result Array**: Create an empty array `$result` to store the filtered elements. 2. **Iterate Through Key List**: Loop through the `$b` array, taking one key to match at a time. 3. **Check for Key Existence**: Inside the loop, use `isset($a[$key_to_match])` to check if the current key exists in array `$a`. `isset()` is generally more efficient than `array_key_exists()` as it also checks if the value is not `null`. 4. **Assign Value**: If the key exists, copy the corresponding key-value pair from `$a` to the `$result` array. #### Code Implementation ```php $result = []; foreach ($b as $key_to_match) { if (isset($a[$key_to_match])) { $result[$key_to_match] = $a[$key_to_match]; } } print_r($result); ``` The output of this method is identical to Method 1. --- ## Comparison and Conclusion | Feature | `array_intersect_key` | `foreach` Loop | | :-------------- | :---------------------------------------------------- | :----------------------------------------------------- | | **Performance** | **Superior**. Built-in functions are usually C-based and faster. | Relatively slower, especially with large datasets. | | **Conciseness** | **Very concise**, solving the problem in one core line. | More verbose, but logic is explicit and easy to debug. | | **Readability** | Highly readable for developers familiar with PHP array functions. | Very friendly and easy to understand for all skill levels. | **Professional Recommendation:** In real-world commercial or open-source projects (like `DP@lib00`), **Method 1 (`array_intersect_key`) is strongly recommended**. It not only performs better but also results in cleaner code, demonstrating a developer's proficiency with PHP's built-in functions, making it the more professional choice.