Extract installation prerequisites into composable classes and use those in the web-based installer

src/Install/Prerequisites/PhpVersion.php

@@ -0,0 +1,24 @@
+<?php
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Install\Prerequisites;
+
+class PhpVersion extends AbstractPrerequisite
+{
+    public function check()
+    {
+        if (version_compare(PHP_VERSION, '5.5.0', '<')) {
+            $this->errors[] = [
+                'message' => '<strong>PHP 5.5+</strong> is required.',
+                'detail' => 'You are running version '.PHP_VERSION.'. Talk to your hosting provider about upgrading to the latest PHP version.'
+            ];
+        }
+    }
+}