Add PHP unit tests and page tests.

Run "composer install", then "composer test".

tests/php/environment.php

@@ -0,0 +1,25 @@
+<?php
+/*
+ * The test environment without the site libraries: a fake database, a
+ * test configuration and an error log that tests can read. The bootstrap
+ * loads it, and so do subprocesses that include a library themselves.
+ */
+if (!defined('XXX_ROOT'))
+  define('XXX_ROOT', dirname(__DIR__, 2));
+require_once XXX_ROOT . '/vendor/autoload.php';
+
+// The site reads the test configuration, never a real .env
+putenv('XXX_ENV_FILE=' . XXX_ROOT . '/tests/php/fixtures/test.env');
+
+// Messages that the code logs go to a file that tests can read
+define('XXX_TEST_LOG', getenv('XXX_TEST_LOG') ?: sys_get_temp_dir() . '/xxx-test-' . getmypid() . '.log');
+ini_set('error_log', XXX_TEST_LOG);
+
+// open_db() makes a FakeDb, so the mysqli extension is not needed
+if (!function_exists('mysqli_report')) {
+  define('MYSQLI_REPORT_OFF', 0);
+  function mysqli_report($flags) { return TRUE; }
+}
+$GLOBALS['db_class'] = Xxx\Tests\Support\FakeDb::class;
+
+$_SERVER['REMOTE_ADDR'] = '192.0.2.1';