<?php
$arr = array(array("id" => 1, "name" => "James"), array("id" => 2, "name" => "Iris"), array("id" => 3, "name" => "Mamá", "nickname" => "Difícil"));
$str = json_encode($arr);
echo $str;
?>
[{"id":1,"name":"James"},{"id":2,"name":"Iris"},{"id":3,"name":"Mam\u00e1","nickname":"Dif\u00edcil"}]