Example of this function output:
<?php
$parse = mailparse_msg_parse_file('mail.txt');
print_r(mailparse_msg_get_structure($parse);
?>
Output will be:
Array
(
[0] => 1
[1] => 1.1
[2] => 1.1.1
[3] => 1.1.2
[4] => 1.2
)