First running version (#2)

This currently depends of interacting with matrix_synapse_rest_auth[1] and mxisd[2].
How to integrate that is explained in the README

[1] https://github.com/kamax-io/matrix-synapse-rest-auth
[2] https://github.com/kamax-io/mxisd

internal/directory_search.php

@@ -0,0 +1,36 @@
+<?php
+require_once("../database.php");
+$response=[
+    "limited" => false,
+    "result" => [],
+];
+
+try {
+    $inputJSON = file_get_contents('php://input');
+    $input = json_decode($inputJSON, TRUE);
+    if (empty($input)) {
+	    throw new Exception('no valid json as input present');
+    }
+    if (!isset($input["by"])) {
+        throw new Exception('"by" is not defined');
+    }
+    if (!isset($input["search_term"])) {
+        throw new Exception('"search_term" is not defined');
+    }
+    switch ($input["by"]) {
+        case "name":
+            $response["result"] = $mx_db->searchUserByName($input["search_term"]);
+            break;
+        case "threepid":
+            $response["result"] = $mx_db->searchUserByEmail($input["search_term"]);
+            break;
+        default:
+            throw new Exception("unknown type for \"by\" param");
+    }
+    
+} catch (Exception $e) {
+    error_log("failed with error: " . $e->getMessage());
+    $response["error"] = $e->getMessage();
+}
+print (json_encode($response, JSON_PRETTY_PRINT) . "\n");
+?>