Fix PHP notices in boot, index and App

- Use defaults() with $_GET variables in boot
- Initialize $a before checking for object member in boot
- Initilialize the $_SESSION variable for backend mode in index
- Add member variable page_offset to App

boot.php

@@ -1635,14 +1635,11 @@ function argv($x)
 function infinite_scroll_data($module)
 {
 	if (PConfig::get(local_user(), 'system', 'infinite_scroll')
-		&& ($module == "network") && ($_GET["mode"] != "minimal")
+		&& $module == 'network'
+		&& defaults($_GET, 'mode', '') != 'minimal'
 	) {
 		// get the page number
-		if (is_string($_GET["page"])) {
-			$pageno = $_GET["page"];
-		} else {
-			$pageno = 1;
-		}
+		$pageno = defaults($_GET, 'page', 1);
 
 		$reload_uri = "";
 
@@ -1653,7 +1650,8 @@ function infinite_scroll_data($module)
 			}
 		}
 
-		if (($a->page_offset != "") && ! strstr($reload_uri, "&offset=")) {
+		$a = get_app();
+		if ($a->page_offset != "" && !strstr($reload_uri, "&offset=")) {
 			$reload_uri .= "&offset=" . urlencode($a->page_offset);
 		}
 

index.php

@@ -97,6 +97,7 @@ if (!$a->is_backend()) {
 	session_start();
 	$a->save_timestamp($stamp1, "parser");
 } else {
+	$_SESSION = [];
 	Worker::executeIfIdle();
 }
 
@@ -148,7 +149,7 @@ if ((x($_GET, 'zrl')) && (!$install && !$maintenance)) {
 
 // header('Link: <' . System::baseUrl() . '/amcd>; rel="acct-mgmt";');
 
-if (x($_COOKIE["Friendica"]) || (x($_SESSION, 'authenticated')) || (x($_POST, 'auth-params')) || ($a->module === 'login')) {
+if (x($_COOKIE, "Friendica") || (x($_SESSION, 'authenticated')) || (x($_POST, 'auth-params')) || ($a->module === 'login')) {
 	require "include/auth.php";
 }
 

src/App.php

@@ -37,6 +37,7 @@ class App {
 	public $query_string;
 	public $config;
 	public $page;
+	public $page_offset;
 	public $profile;
 	public $profile_uid;
 	public $user;