(base) Give a hint in web interface if SimpleXML (php-xml) is missing

Fixes #2180
And prevents #31473, #23970, #18610, #15708

Avoids a 500 error and also gives a useful error message on the web interface if this module isn't installed, gets overlooked during a PHP upgrade, etc.

While we check for it later, it's too late for session.

Inspired by #17163

Signed-off-by: Josh Richards <josh.t.richards@gmail.com>

lib/base.php

@@ -652,6 +652,13 @@ class OC {
 		$bootstrapCoordinator->runInitialRegistration();
 
 		$eventLogger->start('init_session', 'Initialize session');
+
+		// Check for PHP SimpleXML extension earlier since we need it before our other checks and want to provide a useful hint for web users
+		// see https://github.com/nextcloud/server/pull/2619
+		if (!function_exists('simplexml_load_file')) {
+			throw new \OCP\HintException('The PHP SimpleXML/PHP-XML extension is not installed.', 'Install the extension or make sure it is enabled.');
+		}
+		
 		OC_App::loadApps(['session']);
 		if (!self::$CLI) {
 			self::initSession();