<?php

$since = "1090393848";

$params = array("forum"=>"my_forum","since"=>(int)$since,"limit"=>100);

var_dump($params); 

$jsonParams = json_encode($params);

var_dump($jsonParams); 

?>

array(3) {
  ["forum"]=>
  string(8) "my_forum"
  ["since"]=>
  int(1090393848)
  ["limit"]=>
  int(100)
}
string(51) "{"forum":"my_forum","since":1090393848,"limit":100}"