arrays - Get all keys for duplicate values using PHP

Question

I have a PHP array like below

Array
(
    [825] => ca_knife_features 
    [838] => ca_knife_features 
    [795] => ca_knife_handle_materials 
    [853] => ca_knife_handle_materials 
)

and I want to combine all the keys for a same values and make the value as key as below

Array
(
[ca_knife_features]=>array(825,838)
[ca_knife_handle_materials]=>array(795,853)
)

How can I do this?

Answer 1

Getting all the keys for all the values is easy.

foreach ($array as $key => $value) {
    $result[$value][] = $key;
}

But that doesn't get only the duplicate values. There are different ways to do that. One way is by filtering the result to only show values with more than one key.

$result = array_filter($result, function($item) {
    return count($item) > 1;
});