<?php

$data = json_decode('{"search_id":"QkToFW5hdMm2wgXVxx3CnPtr", "trip_id":"6568404916181815103", "customer":{"phone":"+79859731438", "email":"staltip@mail.ru", "name":"Лаврик Валерий"}, "booking":{"status":"booked"}, "passengers":[{"document_type":"passport", "document_number":"4611123123", "first_name":"Валерий", "second_name":"Васильевич", "last_name":"Лаврик", "gender":"M", "birthday":"1990-12-11", "citizenship":"RU", "insurance_id":"0", "insurance_cost":100, "seat_numbers":["18"]}]}');

$s_k = 'y2G7p9xGKXJDy4LhMz4tzn1Z';

$sha256 = hash('sha256', $s_k . json_encode((object)$data) . $s_k);

echo base64_encode($sha256);