contacts = api.contact.list_from_segment(
    list_id=<list ID>,
    segment_id=<segment ID>,
    page=1,
    per_page=50,
    with_count=false
)

<?php

$contacts = $api->contact->listFromSegment([
    'list_id' => <list ID>,
    'segment_id' => <segment ID>,
    'page' => 1,
    'per_page' => 50,
    'with_count' => false
]);