<?php
$items = array(
  0 => array('s'=>1, 'f'=>'janedoe', 'm'=>'dqwdqwd'),
  1 => array('s'=>1, 'f'=>'janedoe', 'm'=>'sdfwsdfgdfwfwe'),
  2 => array('s'=>1, 'f'=>'janedoe', 'm'=>'werwefgwefwefwefweg')
);

echo json_encode(array(
        'username'=>'johndoe', 
        'items'=>$items
    ));
?>

{"username":"johndoe","items":[{"s":1,"f":"janedoe","m":"dqwdqwd"},{"s":1,"f":"janedoe","m":"sdfwsdfgdfwfwe"},{"s":1,"f":"janedoe","m":"werwefgwefwefwefweg"}]}