What does the number signify in `object(stdClass)#12`?

I was dealing with some JSON that I got from a GET request in PHP. I did var_dump(json_decode($response));. My response was something like this:

object(stdClass)#15 (4) {
  ["offset"]=>
  int(0)
  ["rsMailboxes"]=>
  array(6) {
    [0]=>
    object(stdClass)#10 (2) {
      ["name"]=>
      string(7) "pbadamo"
      ["displayName"]=>
      string(15) "weldingworkshop"
    }
    [1]=>
    object(stdClass)#9 (2) {
      ["name"]=>
      string(4) "rick"
      ["displayName"]=>
      string(4) "rick"
    }
    [2]=>
    object(stdClass)#13 (2) {
      ["name"]=>
      string(6) "summer"
      ["displayName"]=>
      string(5) "morty"
    }
    [3]=>
    object(stdClass)#14 (2) {
      ["name"]=>
      string(9) "testemail"
      ["displayName"]=>
      string(9) "testemail"
    }
    [4]=>
    object(stdClass)#12 (2) {
      ["name"]=>
      string(8) "theflash"
      ["displayName"]=>
      string(8) "theflash"
    }
    [5]=>
    object(stdClass)#16 (2) {
      ["name"]=>
      string(9) "tonystark"
      ["displayName"]=>
      string(7) "ironman"
    }
  }
  ["size"]=>
  int(50)
  ["total"]=>
  int(6)
}

What are the numbers after the #? My first thought was that it was a stdClass object ID or something for the shell, but I ran the script again and...the numbers were the same! Could my script have other instances of stdClass before it perhaps? Is that why it starts at #10?