How to detect duplicate values in PHP array?

You can use array_count_values function

$array = array('apple', 'orange', 'pear', 'banana', 'apple',
'pear', 'kiwi', 'kiwi', 'kiwi');

print_r(array_count_values($array));

will output

Array
(
   [apple] => 2
   [orange] => 1
   [pear] => 2
   etc...
)

You could try turning that array into a associative array with the fruits as keys and the number of occurrences as values. Bit long-winded, but it looks like:

$array = array('apple', 'orange', 'pear', 'banana', 'apple',
   'pear', 'kiwi', 'kiwi', 'kiwi');
$new_array = array();
foreach ($array as $key => $value) {
    if(isset($new_array[$value]))
        $new_array[$value] += 1;
    else
        $new_array[$value] = 1;
}
foreach ($new_array as $fruit => $n) {
    echo $fruit;
    if($n > 1)
        echo "($n)";
    echo "<br />";
}

Stuff them into a map (pseudocode)

map[string -> int] $m
foreach($word in $array)
{
    if(!$m.contains($word))
        $m[$word] = 0;

    $m[$word] += 1;
}

Perhaps something like this (untested code but should give you an idea)?

$new = array();

foreach ($array as $value)
{
    if (isset($new[$value]))
        $new[$value]++;
    else
        $new[$value] = 1;
}

Then you'll get a new array with the values as keys and their value is the number of times they existed in the original array.

$data = ['outer', 'inner', 'sole', 'sole', 'outer', 'outer'];

$result = max(array_count_values($data));

if($result > 1) {
  echo 'Duplicate items were found!';
}

I think this way is shorter and cleaner.

function array_not_unique(array $array): array
    {
        $duplicate_array = array_diff_key( $array , array_unique( $array ) );
        $unique_array = [];
        foreach ($array as $key => $value) {
            if ( in_array($value, $duplicate_array)) {
                $duplicate_array[$key] = $value;
            }
            else {
                $unique_array[$key] = $value;
            } 

        }

        return ["unique_array" => $unique_array, "duplicate_array" => $duplicate_array];
    }

This function give you the redundant values only

function array_find_redundant($A){
    $U=$N=[];
    foreach($A As $k=>$v){
        if(in_array($v,$U)){$N[$k]=$v;}else{$U[]=$v;}
    }
    return $N;
}

$A = ['A','B','B','C','C','C'];
$B = array_find_redundant($A); // [2=>'B',4=>'C',5=>'C'] 

A simple method:

$array = array_values(array_unique($array, SORT_REGULAR));