Maybe I misunderstand your request, but it doesn't look hard to
add the part numbers yourself:

<?php
function add_part_numbers($structure, $acc = '') {
    if (isset ($structure->parts)) {
        foreach ($structure->parts as $i => $part) {
            $part_number = $acc . ($i + 1);
            $part->part_number = $part_number;
            add_part_numbers($part, "$part_number.");
        }
    }
}

$structure = imap_fetchstructure($imap, 1);
add_part_numbers($structure);
?>

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