301 Moved Permanently vie PHP header

header.php

<?php
function redirectTo( $location ) {
    header( 'HTTP/1.1 301 Moved Permanently' );
    header( 'Location: ' . $location );
    header( 'Connection: close' );
}
switch ( $_SERVER['REQUEST_URI'] ) {
    case '/post/an-uri-on-old-domain':
      redirectTo( 'https://new-domain.com/a-new-uri/' );
      break;
    case '/post/another-uri-on-old-domain':
      redirectTo( 'https://new-domain.com/another-new-uri/' );
      break;
    ...
    default:
      break;
}