id       | post          | userId   |
---------|---------------|----------|
1        | "hello"       |  1       |
2        | "world"       |  1       |
3        | "ouch"        |  2       |
4        | "test"        |  1       |

id       | postId        | fileName |  time    |
---------|---------------|----------|----------|
1        | 1             |"hey.jpg" |        0 |
2        | 1             |"test.png"| 53252354 |
3        | 2             |"asd.png" |        0 |
4        | 4             |"asd2.png"|        0 |

$qry = $db->prepare('SELECT post.id, post.post, post.userId, att.fileName, att.time
FROM posts AS post LEFT JOIN attachments AS att ON att.postId = post.id');
$qry->execute();
$postsArray = $qry->fetchAll(PDO::FETCH_ASSOC);

[{'id': 1,
'post': 'hello',
'userId': 1,
'attachments': [{'fileName': 'hey.jpg', 'time:' 0}, ... ]
}, ... ]