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/identity_single.php

@@ -0,0 +1,46 @@
+<?php
+require_once("../database.php");
+$response = new stdClass;
+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["lookup"])) {
+        throw new Exception('"lookup" is not defined');
+    }
+    if (!isset($input["lookup"]["medium"])) {
+        throw new Exception('"lookup.medium" is not defined');
+    }
+    if (!isset($input["lookup"]["address"])) {
+        throw new Exception('"lookup.address" is not defined');
+    }
+    $res2 = array();
+    switch ($input["lookup"]["medium"]) {
+        case "email":
+            $res2 = $mx_db->searchUserByEmail($input["lookup"]["address"]);
+            if (!empty($res2)) {
+                $response = [
+                    "lookup" => [
+                        "medium" => $input["lookup"]["medium"],
+                        "address" => $input["lookup"]["address"],
+                        "id" => [
+                            "type" => "localpart",
+                            "value" => $res2[0]["user_id"],
+                        ]
+                    ]
+                ];
+            }
+            
+
+            break;
+        default:
+            throw new Exception("unknown type for \"by\" param");
+    }
+} catch (Exception $e) {
+    error_log("ídentity_bulk failed with error: " . $e->getMessage());
+    $response["error"] = $e->getMessage();
+}
+print (json_encode($response, JSON_PRETTY_PRINT) . "\n");
+?>