デコード、変更、再エンコード。
<?php
$json = '[{"id":"AT.02708872.T"},{"id":"DE.60232348.A"}]';
// Decode the JSON data into an array of objects.
// Symfony probably will have some JSON handling methods so you could look at
// those to keep the code more Symfony friendly.
$array = json_decode($json);
// Loop through the array of objects so you can modify the ID of
// each object. Note the '&'. This is calling $object by reference so
// any changes within the loop will persist in the original array $array
foreach ($array as &$object)
{
// Here we are stripping the periods (.) from the ID and then removing the last
// character with substr()
$object->id = substr(str_replace('.', '', $object->id), 0, -1);
}
// We can now encode $array back into JSON format
$json = json_encode($array);
var_dump($json);
おそらくSymfony2内にネイティブJSON処理があるので、それをチェックすることをお勧めします。