Get the keys for duplicate values in an array

Question

I have the following array:

$myarray = Array("2011-06-21", "2011-06-22", "2011-06-22", "2011-06-23", "2011-06-23", "2011-06-24", "2011-06-24", "2011-06-25", "2011-06-25", "2011-06-26");
var_dump($myarray);

Result:

Array (
    [0] => 2011-06-21
    [1] => 2011-06-22
    [2] => 2011-06-22
    [3] => 2011-06-23
    [4] => 2011-06-23
    [5] => 2011-06-24
    [6] => 2011-06-24
    [7] => 2011-06-25
    [8] => 2011-06-25
    [9] => 2011-06-26
)

1. Now how can I display the keys with duplicate values? Here the function should NOT return ([0],[9]) since there are no duplicates with the values.
2. How to find the keys for the same value, eg. for "2011-06-25" it should return [7],[8]

Answer 1

I'll answer the second question first. You want to use array_keys with the "search_value" specified.

$keys = array_keys($array, "2011-06-29")

In the example below, $duplicates will contain the duplication values while $result will contain ones that are not duplicates. To get the keys, simply use array_keys.

<?php

$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));

Result:

// $duplicates
Array
(
    [1] => a
)

// $result
Array
(
    [2] => b
    [3] => c
    [4] => d
)

// $unique_keys
Array
(
    [0] => 2
    [1] => 3
    [2] => 4
)

// $duplicate_keys
Array
(
    [0] => 0
    [1] => 1
)