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

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.