问如何在PHP中通过值获取所有键的数组？

EN

$details = ["Dog" => "Natahlia", "Cat" => "Ryan", "Bird" => "Natahlia"];

$details = [["Natahlia"=>["Dog","Bird"], "Ryan"=>"Cat"];

$arr = array();
foreach($details as $key => $value){
   if(array_key_exists($value,$arr)){
      //this part i dont know what to do since += wont do it since it's a string and .= concatenates it
      $arr[$value] = $key;
   } else {
      $arr[$value] = $key;
   }
}
return $arr;