chromium/src/third_party/blink/web_tests/http/tests/misc/resources/image-checks-for-viewport-width.php

<?php
    $vw = $_SERVER["HTTP_VIEWPORT_WIDTH"];
    $expected_vw = $_GET["viewport"];

    if ((isset($expected_vw) && $vw == $expected_vw) || (isset($vw) && !isset($expected_vw))) {
        $fn = fopen("compass.jpg", "r");
        fpassthru($fn);
        fclose($fn);
        exit;
    }
    header("HTTP/1.1 417 Expectation failed");
?>