(PHP 8 >= 8.1.0)
ReflectionEnum::getBackingType — Gets the backing type of an Enum, if any
   If the enumeration is a Backed Enum, this method will return an instance
   of ReflectionType for the backing type of the Enum.
   If it is not a Backed Enum, it will return null.
  
This function has no parameters.
   An instance of ReflectionType, or null
   if the Enum has no backing type.
  
Example #1 ReflectionEnum::getBackingType() example
<?php
enum Suit: string
{
    case Hearts = 'H';
    case Diamonds = 'D';
    case Clubs = 'C';
    case Spades = 'S';
}
$rEnum = new ReflectionEnum(Suit::class);
$rBackingType = $rEnum->getBackingType();
var_dump((string)$rBackingType);
?>
The above example will output:
string(6) "string"