Don't call session_start() after ini_set()

refs #3185

library/Icinga/Web/Session.php

@@ -29,7 +29,7 @@ class Session
     public static function create(BaseSession $session = null)
     {
         if ($session === null) {
-            self::$session = new PhpSession();
+            self::$session = PhpSession::create();
         } else {
             self::$session = $session;
         }

library/Icinga/Web/Session/Php72Session.php

@@ -0,0 +1,37 @@
+<?php
+/* Icinga Web 2 | (c) 2017 Icinga Development Team | GPLv2+ */
+
+namespace Icinga\Web\Session;
+
+use Icinga\Application\Logger;
+use Icinga\Exception\ConfigurationError;
+use Icinga\Web\Cookie;
+
+/**
+ * Session implementation in PHP
+ */
+class Php72Session extends PhpSession
+{
+    /**
+     * Open a PHP session
+     */
+    protected function open()
+    {
+        session_name($this->sessionName);
+
+        $cookie = new Cookie('bogus');
+        session_set_cookie_params(
+            0,
+            $cookie->getPath(),
+            $cookie->getDomain(),
+            $cookie->isSecure(),
+            true
+        );
+
+        session_start(array(
+            'use_cookies'       => true,
+            'use_only_cookies'  => true,
+            'use_trans_sid'     => false
+        ));
+    }
+}

library/Icinga/Web/Session/PhpSession.php

@@ -33,6 +33,21 @@ class PhpSession extends Session
      */
     protected $sessionName = 'Icingaweb2';
 
+    /**
+     * Create a new PHPSession object using the provided options (if any)
+     *
+     * @param   array   $options    An optional array of ini options to set
+     *
+     * @return  static
+     *
+     * @throws  ConfigurationError
+     * @see     http://php.net/manual/en/session.configuration.php
+     */
+    public static function create(array $options = null)
+    {
+        return version_compare(PHP_VERSION, '7.2.0') < 0 ? new self($options) : new Php72Session($options);
+    }
+
     /**
      * Create a new PHPSession object using the provided options (if any)
      *

test/php/library/Icinga/Web/Session/PhpSessionTest.php

@@ -13,7 +13,7 @@ class PhpSessionTest extends BaseTestCase
         if (!is_writable('/tmp')) {
             $this->markTestSkipped('Could not write to session directory');
         }
-        return new PhpSession(
+        return PhpSession::create(
             array(
                 'use_cookies'   => false,
                 'save_path'     => '/tmp',