<?php
aoeu
if (!isset($_POST['photoSrc']) || !isset($_POST['caption']) ) {
    header("HTTP/1.1 400 Bad Request need both 'photoSrc' and 'caption'");
    die("Unable to save caption without photo and caption");
}

// Not that great to just store data in the same directory as the web service, but 
// we don't have any way to protect our data just yet...
$datafile = "data.txt";

$jsonEncoded = json_encode([
    'photoSrc' => $_POST['photoSrc'],
    'caption' => $_POST['caption'],
]);

file_put_contents($datafile, $jsonEncoded, FILE_APPEND);

print(json_encode(["status" => "success"]));