Notes on using external variables in custom comparison functions

array_diff_uassoc is a function used in PHP to compare two arrays. It not only compares the values ​​of the array, but also compares the keys of the array. In some cases, we may want to use a custom comparison function when comparing. This custom comparison function can often add additional logic when comparing array values ​​and keys. However, if you need to use external variables in comparison functions (such as global variables or parameters passed to the function), you need to pay special attention to how to correctly refer to these external variables in a custom function.

This article will explain how to correctly use external variables in custom comparison functions when using array_diff_uassoc .

1. What is array_diff_uassoc ?

array_diff_uassoc is an array function provided by PHP that takes multiple arrays as parameters and returns a new array containing elements in the first array but not in other arrays. It is similar to array_diff_assoc , but allows the user to provide a custom comparison function for comparing keys to arrays.

The function prototype is as follows:

 array array_diff_uassoc ( array $array1 , array $array2 , array ...$arrays , callable $key_compare_func )

• $array1 is the first array used for comparison.

• $array2, ...$arrays is one or more arrays for comparison.

• $key_compare_func is a custom callback function used to compare keys of arrays.

2. Customize external variables in comparison function

When writing custom comparison functions, we sometimes need to use external variables (such as some global settings or parameters from outside the function). However, in PHP, anonymous functions (closures) cannot directly access externally scoped variables unless we explicitly pass these variables to the function.

3. Example: How to use external variables in array_diff_uassoc

Suppose we have two arrays, $array1 and $array2 , we want to compare the keys of these two arrays and use some externally defined variable (for example, a set threshold) during the comparison. We will use an example to demonstrate how to do it.

Sample code:

 <?php
// External variables：A threshold
$threshold = 5;

// Custom comparison functions
function compare_keys($a, $b) {
    global $threshold; // Reference external global variables $threshold
    
    // If the two keys are equal，return 0
    if ($a === $b) {
        return 0;
    }

    // Comparison using external thresholds
    return abs($a - $threshold) < abs($b - $threshold) ? -1 : 1;
}

// Comparison of two arrays
$array1 = [3 => 'apple', 7 => 'banana', 10 => 'cherry'];
$array2 = [2 => 'pear', 7 => 'grape', 9 => 'orange'];

// use array_diff_uassoc And pass in a custom comparison function
$result = array_diff_uassoc($array1, $array2, 'compare_keys');

print_r($result);
?>

Code parsing:

1. Global variable $threshold : We define an external $threshold variable to set a "reference" value when we compare keys.

2. Custom comparison function compare_keys : This function accepts two parameters $a and $b , representing the two keys being compared. We use global $threshold to enable this function to access the external $threshold variable. In the function, we calculate the distance between the key and $threshold and return the comparison result.

3. Use of array_diff_uassoc : When calling array_diff_uassoc , we pass the arrays $array1 and $array2 as parameters and pass in the custom comparison function compare_keys . This function is automatically called when comparing array keys.

4. Output result : Print out the result through print_r , you can see which elements are in $array1 but not in $array2 , and are based on our customized comparison rules.

4. Summary

When using PHP's array_diff_uassoc , if you need to use external variables in a custom comparison function, you can implement it in the following ways:

• Reference global variables using global keywords.

• Pass external variables as parameters to a custom comparison function (for example, using the use keyword to introduce external variables in an anonymous function).

• Ensure that the return value of the comparison function meets the requirements of array_diff_uassoc and correctly handles the key comparison logic.

Hopefully this example helps you understand how to use external variables correctly in array_diff_uassoc and be handy in actual development.