Check and return duplicates array php

Question

I would like to check if my array has any duplicates and return the duplicated values in an array. I want this to be as efficient as possible.

Example:

Answer 1

You can get the difference of the original array and a copy without duplicates using array_unique and array_diff_assoc:

array_diff_assoc($arr, array_unique($arr))

Answer 2

function returndup($array) 
{
    $results = array();
    $duplicates = array();
    foreach ($array as $item) {
        if (in_array($item, $results)) {
            $duplicates[] = $item;
        }

        $results[] = $item;
    }

    return $duplicates;
}

Answer 3

in addition to gumbo's answer:

function returndup($arr)
{
  return array_diff_key($arr, array_unique($arr));
}

Answer 4

I did some tests and indeed @user187291's variant is the fastest. But, it turns out that @Gumbo's and @faebser's alternative are almost as fast, @faebser's being just slightly faster than @Gumbo's and sometimes even fastest of all.

Here's the code I used

$array = array(1, "hello", 1, "world", "hello");
$times = 1000000;

$start = microtime(true);
for ($i = 0; $i < $times; $i++) {
    $dups = array();
    foreach(array_count_values($array) as $val => $c)
        if( $c > 1) $dups[] = $val;
}
$end = microtime(true);

echo 'variant 1 (user187291): ' . ($end - $start);
echo '<br><br><br>';

$start = microtime(true);
for ($i = 0; $i < $times; $i++)
    $dups = array_unique(array_diff_assoc($array, array_unique($array)));
$end = microtime(true);

echo 'variant 2 (JAL): ' . ($end - $start);
echo '<br><br><br>';

$start = microtime(true);
for ($i = 0; $i < $times; $i++)
    $dups = array_diff_assoc($array, array_unique($array));
$end = microtime(true);

echo 'variant 3 (Gumbo): ' . ($end - $start);
echo '<br><br><br>';

$start = microtime(true);
for ($i = 0; $i < $times; $i++)
    $dups = array_diff_key($array, array_unique($array));
$end = microtime(true);

echo 'variant 4 (faebser): ' . ($end - $start);
echo '<br><br><br>';