Why does the first array parameter play an important role as the main array in the array_diff_uassoc function?

M66 2025-06-06

In PHP, array_diff_uassoc() is a very powerful array function that can return elements with different key names in two arrays based on user-defined comparison functions. The first parameter of the function is usually called the main array, which plays a very important role throughout the operation. So, why does the first array parameter play an important role as the main array in the array_diff_uassoc() function?

Introduction to array_diff_uassoc Function

The function of the array_diff_uassoc() function is to compare the key values of two or more arrays and return elements with different key names. The comparison is performed through the user-provided callback function. The function signature is as follows:

 array_diff_uassoc ( array $array1 , array $array2 [, array $... ], callable $key_compare_func ) : array

$array1 : The main array, the first parameter, it is the array to be compared.
$array2, ... : Other arrays to be compared.
$key_compare_func : A user-defined comparison function for comparing the key names of an array.

Why is the first parameter the main array?

The benchmark role of the main array
The first parameter (main array) of array_diff_uassoc() plays a benchmark role in the comparison process. When executing a function, each key name in the main array is compared with the key name of the other arrays. Ultimately, the returned key-value pairs that exist in the main array but do not exist in other arrays. Therefore, the main array determines the content of the result.
The main array determines the returned key-value pair
The purpose of the comparison function is to return key-value pairs that exist in the main array but cannot be found in other arrays. Regardless of the result of the comparison, only the elements in the main array will appear in the final return result. If elements in the main array have the same key names in other arrays and comply with the comparison rules, they are excluded from the result.
The role of custom comparison functions
The user-defined comparison function ( key_compare_func ) is used to compare key names, which will determine whether the key names of the two arrays are equal. This comparison function usually receives two parameters: the key in the main array and the key in the other arrays. Through this comparison function, we can judge whether the key names are equal according to specific rules. Each key name in the main array is compared to the key name in the other arrays.
Sequence question
Another aspect to note is that array_diff_uassoc() compares the order of the main array and whether other arrays contain elements with the same key name. If the key-value pairs of the two arrays are in the same order, but the key names in the main array are not found in other arrays, it will still be treated as different elements.

Examples in practical applications

Let's show how array_diff_uassoc() works with a simple example. Suppose we have two arrays and we want to find elements that exist in the first array but not found in the second array.

 <?php

// Define the main array and another array
$array1 = [
    'a' => 1,
    'b' => 2,
    'c' => 3
];

$array2 = [
    'a' => 1,
    'b' => 4
];

// Custom comparison functions：Used to compare key names
function compare_keys($key1, $key2) {
    return strcmp($key1, $key2);
}

// use array_diff_uassoc Compare arrays
$result = array_diff_uassoc($array1, $array2, 'compare_keys');

// Output result
print_r($result);

?>

Output result:

 Array
(
    [c] => 3
)

In this example, the main array $array1 contains three key-value pairs, while $array2 has only two key-value pairs. Through the array_diff_uassoc() function, we find the key-value pair 'c' => 3 that is in $array1 but not in $array2 . As you can see, the returned array only has elements that exist in the main array but cannot be found in other arrays.

in conclusion

In general, the first array parameter in array_diff_uassoc() plays a decisive role as the main array, which determines the content of the comparison result. By comparing the key names in the main array with those in other arrays, the ultimate return is an element that exists in the main array but cannot be found in other arrays. Therefore, understanding the role of the main array in this function is crucial to the rational use of the function.

array_diff_uassoc