Here a full example to save ZIP files attachments in their original name.

<?php
$email_raw = '(raw e-mail contents buffer)';
$parser = mailparse_msg_create(); // MUST be destroyed at the end of the script
mailparse_msg_parse($parser, $email_raw);
$structure = mailparse_msg_get_structure($parser); // Ex. ["1", "1.1", "1.2"]
foreach ($structure as $part_label) { // Search among each e-mail part
    $part = mailparse_msg_get_part($parser, $part_label); // Parse a specified part
    $part_data = mailparse_msg_get_part_data($part); // Get parsed part data, header and meta values
    if ($part_data['content-type'] ?? null === 'application/zip') {
        $name = $part_data['disposition-filename'] ?? $part_data['content-name'] ?? 'unknow.zip';
        $contents = mailparse_msg_extract_part($part, $email_raw, null); // null for returning content
        file_put_contents($name, $contents);
    }
}
mailparse_msg_free($parser); // Important

With ref to previous comment re: callback:

If you explicitly specify NULL as the callback parameter, the complete section is extracted, decoded and returned, without the need for a callback.

In mailparse version 2.1.1 (and perhaps earlier), when using mailparse_msg_extract_part() with a callback function, it breaks the data it passes to it into 4kB chunks and calls the callback function for each chunk.  So, for example, if it's extracting a 41kB MIME part, the callback function you define will be called 11 times, each time with the next chunk of data.  Here's some quick-and-dirty code that shows one way to handle this:

<?php
    $message = file_get_contents ("email.txt"); // Pull in the e-mail.

    function catch_part ($part)
    {
        $GLOBALS["part_data"] .= $part; // Append the data onto any previously extracted data.
    }

    mailparse_msg_extract_part ("1.1", $message, "catch_part"); // Extract MIME part 1.1
    echo $GLOBALS["part_data"]; // Print out the extracted part.
?>

There's probably a much better way of dealing with this, but hey.  It's what I got.

substr() uses the string length, not the position as third argument. The corrected version of the following code line:
<?php
$parts[$s] = substr($file_txt, $starting_pos_body, $ending_pos_body-$starting_pos_body);
?>

The callback argument does not support closures... :( It will complains with "PHP Catchable fatal error:  Object of class Closure could not be converted to string".

Unless I've missed something obvious:

get_structure returns array(1,1.1,1.1.2) etc but its not easy to get the contents of each part as mailparse_msg_extract_part() and mailparse_msg_extract_part_file() just return the lot.  However get_part_data will return the string offsets so you know where to chop the message so you can get the contents of the parts.

Only issue is get_part_data returns:
    [starting-pos] => 0
    [starting-pos-body] => 1412
    [ending-pos] => 14989
    [ending-pos-body] => 14989

Unless I'm missed something else, theres a bug here as ending-pos is the same as ending-pos-body so it won't chop the contents cleanly, leaving the:

------=_NextPart_000_0069_01C659A6.9072E590--

...as supposedly part of the section contents.

$file = "..../mail"; // path of your mail
$file_txt = implode("",file($file));
$parse = mailparse_msg_parse_file($file); 
$structure = mailparse_msg_get_structure($parse);
// chop message parts into array
$parts = array();
foreach ($structure as $s){
    print "Part $s\n";
    print "--------------------------------------\n";
    $part = mailparse_msg_get_part($parse, $s);
    $part_data = mailparse_msg_get_part_data($part);
    print_r($part_data);
    $starting_pos_body = $part_data['starting-pos-body'];
    $ending_pos_body    = $part_data['ending-pos-body'];
    $parts[$s] = substr($file_txt,$starting_pos_body,$ending_pos_body); // copy data into array
    print "[".$parts[$s]."]";
    print "\n------------------------------------\n";
}