php get keys of duplicate values in array

$arr = array(1, 4, 6, 1, 8, 9, 4, 6);

$unique = array_unique($arr);

$duplicates = array_diff_assoc($arr, $unique);

print_r($duplicates);
Array ( [3] => 1 [6] => 4 [7] => 6 )

$arr = array(3,5,2,5,3,9);
foreach($arr as $key => $val){
  //remove the item from the array in order 
  //to prevent printing duplicates twice
  unset($arr[$key]); 
  //now if another copy of this key still exists in the array 
  //print it since it's a dup
  if (in_array($val,$arr)){
    echo $val . " ";
  }
}

<?php
/** 
	Examples of these functions:
	array_unique, 
    array_diff_assoc, 
    array_diff, 
    array_keys, 
    array_intersect 
    
    Examle with an array: 
*/
$array = array('a', 'a', 'b', 'c', 'd');

// Unique values
$unique = array_unique($array);

// Duplicates
$duplicates = array_diff_assoc($array, $unique);

// Unique values
$result = array_diff($unique, $duplicates);

// Get the unique keys
$unique_keys = array_keys($result);

// Get duplicate keys
$duplicate_keys = array_keys(array_intersect($array, $duplicates));