<?php
$arr = array(0 => array('id' => 126, 'type' => 'txt'), 1 => array('id' => 586, 'type' => 'txt'));
$res = array();
foreach ($arr as $k => $v)
{
    $res[] = array("data_{$k}_id" => $v['id'], "data_{$k}_type" => $v['type']);
}
print_r($res);