Why does using array_diff_assoc() not return the expected result?

M66 2025-05-17

In PHP, array_diff_assoc() is a very useful function that compares two arrays, returns all elements in the first array that are not in the second array, preserving the consistency between the key name and the key value. However, sometimes you may find that the result is not what you expected when using array_diff_assoc() . This article will explain the common causes of this problem and help you diagnose and resolve the problem.

Basic use of array_diff_assoc()

The function of array_diff_assoc() is to compare the keys and values of two arrays and return elements in the first array that are different from the second array. Its function prototype is as follows:

 array_diff_assoc(array $array1, array $array2): array

This function will compare $array1 and $array2 , and if the key and value of the elements in $array1 are different from $array2 , these elements will be returned.

Sample code:

 $array1 = [
    "a" => "apple",
    "b" => "banana",
    "c" => "cherry"
];

$array2 = [
    "a" => "apple",
    "b" => "berry",
    "d" => "date"
];

$result = array_diff_assoc($array1, $array2);
print_r($result);

Output result:

 Array
(
    [b] => banana
    [c] => cherry
)

In this example, array_diff_assoc() returns elements in $array1 , with keys b and c , and values are different from those in $array2 .

Frequently Asked Questions

Different keys and values of the array cause the result to not meet expectations
array_diff_assoc() not only compares the values of an array, it also compares the keys. If the keys of the two arrays are exactly the same but the values are different, then these elements will still be considered different. So if you only care about the difference in values, you may need to use the array_diff() function, which only compares the values and does not care about the keys.
For example:
```
 $array1 = ["a" => "apple", "b" => "banana"];
$array2 = ["a" => "apple", "b" => "berry"];

$result = array_diff_assoc($array1, $array2);
print_r($result);
```
Output result:
```
 Array
(
    [b] => banana
)
```
You can see that although the value of b is different, it is treated as a different element due to the same key name.
Array contains nested arrays or objects <br> If you use array_diff_assoc() , the array contains nested arrays or objects, the function will perform recursive comparisons. If these nested elements are not processed correctly or are not compared as expected, you may encounter unexpected results. In this case, it is very important to debug the structure of nested arrays or objects.
Different types of elements
PHP is a loosely typed language, so elements in an array may be of different types. array_diff_assoc() compares values and types, not just values. If the elements of the same position in two arrays are different, the function will consider them different even if their values look the same.
For example:
```
 $array1 = [1 => "10", 2 => "20"];
$array2 = [1 => 10, 2 => 20];

$result = array_diff_assoc($array1, $array2);
print_r($result);
```
Output result:
```
 Array
(
    [1] => 10
    [2] => 20
)
```
"10" and 10 are different here, one is a string and the other is an integer, so array_diff_assoc() thinks they are not the same.
The order of elements in an array <br> Although array_diff_assoc() not only compares values but also includes key names, it takes into account the order of the array. If you pass two arrays to array_diff_assoc() , although their content is the same but in different order, then they will still be considered different arrays.
```
 $array1 = [1 => "apple", 2 => "banana"];
$array2 = [2 => "banana", 1 => "apple"];

$result = array_diff_assoc($array1, $array2);
print_r($result);
```
Output result:
```
 Array
(
    [1] => apple
    [2] => banana
)
```
Due to the different order of arrays, the result returns all elements of array1 .
The keys of the array do not start from 0 <br> In some cases, the keys of the array may not start at 0, which will also affect the result of array_diff_assoc() . Make sure you understand what the keys of the array are and whether you intentionally use discontinuous keys.

Solution

If you don't care about the keys of the array, or you just need to compare values without considering the differences in keys, it is recommended to use array_diff() .
Ensure that the values and key types of the array are consistent and avoid type mismatch.
If there are nested arrays or objects in the array, check if they are compared as expected, or debug in recursive manner.

summary

array_diff_assoc() is a powerful function, but its comparison logic is based on strict matching of keys and values. So if you don't get the expected results when using it, it's usually due to differences in keys, values, or order of the array. Understanding how this function works and ensuring that the keys and values of the array are in line with expectations can avoid most common problems.