PHP array_diff: How it Works with Examples

The array_diff function compares arrays and returns values that exist in the first array but not in the others in PHP. This makes it useful when you need to filter unique items from a group of arrays.

Understand the array_diff Function in PHP

The function checks each value in the first array. It then removes values that also exist in the second array or in the next arrays. The order stays the same as the input array.

Here is the syntax:

array_diff(array $array1, array $array2, array $array3...): array

• The first parameter is the main array.
• The rest are arrays to compare against.
• It returns an array that holds only unique values from the first array.

Here is a quick example:

$first = [1, 2, 3, 4];
$second = [3, 4, 5];
$result = array_diff($first, $second);
print_r($result);

The output:

Array
(
    [0] => 1
    [1] => 2
)

This example gives [1, 2] because 3 and 4 exist in the second array. The function does not compare keys, it only checks values.

When you pass nested arrays, array_diff does not look deep inside. It only checks top values and ignores sub arrays. To compare nested data, you must use another method.

The array_diff with Associative Arrays

Associative arrays also work with array_diff. The function checks only the values and keeps the original keys in the result. Keys do not affect the comparison process.

Here is an example:

$arr1 = [
    "a" => "red",
    "b" => "green",
    "c" => "blue",
    "d" => "yellow"
];

$arr2 = [
    "e" => "red",
    "f" => "blue",
    "g" => "black"
];

$result = array_diff($arr1, $arr2);

print_r($result);

The output:

Array
(
    [b] => green
    [d] => yellow
)

The Differences Between array_diff and array_intersect

The array_diff function returns values that are unique to the first array. The array_intersect function does the opposite and returns values that exist in all arrays.

Here is a table that shows you the key differences:

You use array_diff when you need items excluded. You use array_intersect when you need items included across all arrays.

Examples of the array_diff Function in PHP

Compare Two Arrays of Numbers:

$a = [10, 20, 30, 40];
$b = [30, 40, 50];
$result = array_diff($a, $b);
print_r($result);

The output:

Array
(
    [0] => 10
    [1] => 20
)

This code gives [10, 20]. The result has values that only exist in the first array. You can use this method to remove unwanted numbers from a data set.

Compare Associative Arrays with Values:

$first = ["x" => 100, "y" => 200, "z" => 300];
$second = ["a" => 200, "b" => 400];
$result = array_diff($first, $second);
print_r($result);

The output:

Array
(
    [x] => 100
    [z] => 300
)

The output is [100, 300]. Keys do not matter in this case. The function only compares values and keeps the original key structure.

Use array_diff with Multiple Arrays:

$main = [1, 2, 3, 4, 5, 6];
$x = [2, 4];
$y = [6, 7];
$result = array_diff($main, $x, $y);
print_r($result);

The output:

Array
(
    [0] => 1
    [2] => 3
    [4] => 5
)

The result is [1, 3, 5]. The function removed values that exist in both the second and third arrays. This is useful when you must filter a list against many sets of data.

Wrapping Up

In this article you learned the syntax of array_diff and its use with both indexed arrays and associative arrays. You also saw how it behaves with nested arrays, how it compares to array_intersect, and how it works with multiple arrays.

Here is a quick recap:

• array_diff returns values unique to the first array.
• It ignores keys and only checks values.
• It can compare multiple arrays at once.
• Use array_intersect to find common values instead.

FAQs

$array1 = array("a" => "red", "b" => "green", "c" => "blue");
$array2 = array("a" => "red", "b" => "yellow");

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

Array
(
    [b] => green
    [c] => blue
)

$array1 = array("apple", "banana", "mango", "grape");
$array2 = array("banana", "grape");
$array3 = array("apple");

$result = array_diff($array1, $array2, $array3);
print_r($result);

Array
(
    [2] => mango
)

$array1 = array("red", "green", "blue");
$array2 = array("green", "yellow", "blue");

$diff = array_diff($array1, $array2);
$intersect = array_intersect($array1, $array2);

print_r($diff);
print_r($intersect);

Array
(
    [0] => red
)

Array
(
    [1] => green
    [2] => blue
)

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