Return only duplicated entries from an array (case-insensitive)

Question

I want to retrieve all case-insensitive duplicate entries from an array. Is this possible in PHP?

array(
    1 => '1233',
    2 => '12334',
    3 => 'Hello',
    4 => 'hello',
    5 => 'U'
);

Desired output array:

array(
    1 => 'Hello',
    2 => 'hello'
);

Answer 1

function get_duplicates ($array) {
    return array_unique( array_diff_assoc( $array, array_unique( $array ) ) );
}

Answer 2

<?php
function array_not_unique($raw_array) {
    $dupes = array();
    natcasesort($raw_array);
    reset($raw_array);

    $old_key   = NULL;
    $old_value = NULL;
    foreach ($raw_array as $key => $value) {
        if ($value === NULL) { continue; }
        if (strcasecmp($old_value, $value) === 0) {
            $dupes[$old_key] = $old_value;
            $dupes[$key]     = $value;
        }
        $old_value = $value;
        $old_key   = $key;
    }
    return $dupes;
}

$raw_array    = array();
$raw_array[1] = 'abc@xyz.com';
$raw_array[2] = 'def@xyz.com';
$raw_array[3] = 'ghi@xyz.com';
$raw_array[4] = 'abc@xyz.com'; // Duplicate

$common_stuff = array_not_unique($raw_array);
var_dump($common_stuff);

Answer 3

You will need to make your function case insensitive to get the "Hello" => "hello" result you are looking for, try this method:

$arr = array(1=>'1233',2=>'12334',3 =>'Hello' ,4=>'hello', 5=>'U');

// Convert every value to uppercase, and remove duplicate values
$withoutDuplicates = array_unique(array_map("strtoupper", $arr));

// The difference in the original array, and the $withoutDuplicates array
// will be the duplicate values
$duplicates = array_diff($arr, $withoutDuplicates);
print_r($duplicates);

Output is:

Array
(
[3] => Hello
[4] => hello
)

---

Edit by @AlixAxel:

This answer is very misleading. It only works in this specific condition. This counter-example:

$arr = array(1=>'1233',2=>'12334',3 =>'Hello' ,4=>'HELLO', 5=>'U');

Fails miserably. Also, this is not the way to keep duplicates:

array_diff($arr, array_unique($arr));

Since one of the duplicated values will be in array_unique, and then chopped off by array_diff.

Edit by @RyanDay:

So look at @Srikanth's or @Bucabay's answer, which work for all cases (look for case insensitive in Bucabay's), not just the test data specified in the question.

Answer 4

This is the correct way to do it (case-sensitive):

array_intersect($arr, array_unique(array_diff_key($arr, array_unique($arr))));

And a case-insensitive solution:

$iArr = array_map('strtolower', $arr);
$iArr = array_intersect($iArr, array_unique(array_diff_key($iArr, array_unique($iArr))));

array_intersect_key($arr, $iArr);

But @Srikanth answer is more efficient (actually, it's the only one that works correctly besides this one).

Answer 5

function array_not_unique($raw_array) {
    $dupes = array();
    natcasesort($raw_array);
    reset($raw_array);

    $old_key   = NULL;
    $old_value = NULL;
    foreach ($raw_array as $key => $value) {
        if ($value === NULL) { continue; }
        if (strcasecmp($old_value, $value) === 0) {
            $dupes[$old_key] = $old_value;
            $dupes[$key]     = $value;
        }
        $old_value = $value;
        $old_key   = $key;
    } return $dupes;
}

What Srikanth (john) added but with the case insensitive comparison.

Answer 6

Try:

$arr2 = array_diff_key($arr, array_unique($arr));

---

case insensitive:

array_diff_key($arr, array_unique(array_map('strtolower', $arr)));

Answer 7

12 year old post and the accepted answer returns a blank array and others are long.

Here is my take for future Googlers that is short and returns ALL duplicate indexes (Indices?).

$myArray = array('fantastic', 'brilliant', 'happy', 'fantastic', 'Happy', 'wow', 'battlefield2042 :(');

function findAllDuplicates(array $array)
{
    // Remove this line if you do not need case sensitive.
    $array = array_map('strtolower', $array);

    // Remove ALL duplicates
    $removedDuplicates = array_diff($array, array_diff_assoc($array, array_unique($array)));

    return array_keys(array_diff($array, $removedDuplicates));
    // Output all keys with duplicates
    // array(4) {
    //   [0]=>int(0)
    //   [1]=>int(2)
    //   [2]=>int(3)
    //   [3]=>int(4)
    // }


    return array_diff($array, $removedDuplicates);
    // Output all duplicates
    // array(4) {
    //   [0]=>string(9) "fantastic"
    //   [2]=>string(5) "happy"
    //   [3]=>string(9) "fantastic"
    //   [4]=>string(5) "happy"
    // }
}