diff --git a/phpBB/adm/style/acp_main.html b/phpBB/adm/style/acp_main.html
index b8d337cfd8..0c87ed65b4 100644
--- a/phpBB/adm/style/acp_main.html
+++ b/phpBB/adm/style/acp_main.html
@@ -78,6 +78,13 @@
 				<p>{L_ERROR_MBSTRING_HTTP_OUTPUT_EXPLAIN}</p>
 			</div>
 		<!-- ENDIF -->
+
+		{% if S_DEFAULT_CHARSET_FAIL %}
+			<div class="errorbox">
+				<h3>{{ lang('ERROR_DEFAULT_CHARSET') }}</h3>
+				<p>{{ lang('ERROR_DEFAULT_CHARSET_EXPLAIN') }}</p>
+			</div>
+		{% endif %}
 	<!-- ENDIF -->
 
 	<!-- IF S_WRITABLE_CONFIG -->
diff --git a/phpBB/includes/acp/acp_main.php b/phpBB/includes/acp/acp_main.php
index 5c07cad487..4abc176d19 100644
--- a/phpBB/includes/acp/acp_main.php
+++ b/phpBB/includes/acp/acp_main.php
@@ -672,16 +672,22 @@ class acp_main
 		$encoding_translation	= $this->php_ini->getString('mbstring.encoding_translation');
 		$http_input				= $this->php_ini->getString('mbstring.http_input');
 		$http_output			= $this->php_ini->getString('mbstring.http_output');
+		$default_charset		= $this->php_ini->getString('default_charset');
 
 		if (extension_loaded('mbstring'))
 		{
-			$template->assign_vars(array(
+			/**
+			 * “mbstring.http_input” and “mbstring.http_output” are deprecated as of PHP 5.6.0
+			 * @link https://www.php.net/manual/mbstring.configuration.php#ini.mbstring.http-input
+			 */
+			$template->assign_vars([
 				'S_MBSTRING_LOADED'						=> true,
 				'S_MBSTRING_FUNC_OVERLOAD_FAIL'			=> $func_overload && ($func_overload & (MB_OVERLOAD_MAIL | MB_OVERLOAD_STRING)),
 				'S_MBSTRING_ENCODING_TRANSLATION_FAIL'	=> $encoding_translation && ($encoding_translation != 0),
-				'S_MBSTRING_HTTP_INPUT_FAIL'			=> $http_input && !in_array($http_input, array('pass', '')),
-				'S_MBSTRING_HTTP_OUTPUT_FAIL'			=> $http_output && !in_array($http_output, array('pass', '')),
-			));
+				'S_MBSTRING_HTTP_INPUT_FAIL'			=> !empty($http_input),
+				'S_MBSTRING_HTTP_OUTPUT_FAIL'			=> !empty($http_output),
+				'S_DEFAULT_CHARSET_FAIL'				=> $default_charset !== 'UTF-8',
+			]);
 		}
 
 		// Fill dbms version if not yet filled
diff --git a/phpBB/language/en/acp/common.php b/phpBB/language/en/acp/common.php
index bf1ed5f34f..eda5b6ccf9 100644
--- a/phpBB/language/en/acp/common.php
+++ b/phpBB/language/en/acp/common.php
@@ -390,9 +390,11 @@ $lang = array_merge($lang, array(
 	'ERROR_MBSTRING_ENCODING_TRANSLATION'			=> 'Transparent character encoding is improperly configured',
 	'ERROR_MBSTRING_ENCODING_TRANSLATION_EXPLAIN'	=> '<var>mbstring.encoding_translation</var> must be set to 0. You can check the current value on the <samp>PHP information</samp> page.',
 	'ERROR_MBSTRING_HTTP_INPUT'						=> 'HTTP input character conversion is improperly configured',
-	'ERROR_MBSTRING_HTTP_INPUT_EXPLAIN'				=> '<var>mbstring.http_input</var> must be set to <samp>pass</samp>. You can check the current value on the <samp>PHP information</samp> page.',
+	'ERROR_MBSTRING_HTTP_INPUT_EXPLAIN'				=> '<var>mbstring.http_input</var> must be left empty. You can check the current value on the <samp>PHP information</samp> page.',
 	'ERROR_MBSTRING_HTTP_OUTPUT'					=> 'HTTP output character conversion is improperly configured',
-	'ERROR_MBSTRING_HTTP_OUTPUT_EXPLAIN'			=> '<var>mbstring.http_output</var> must be set to <samp>pass</samp>. You can check the current value on the <samp>PHP information</samp> page.',
+	'ERROR_MBSTRING_HTTP_OUTPUT_EXPLAIN'			=> '<var>mbstring.http_output</var> must be left empty. You can check the current value on the <samp>PHP information</samp> page.',
+	'ERROR_DEFAULT_CHARSET'							=> 'Default charset is improperly configured',
+	'ERROR_DEFAULT_CHARSET_EXPLAIN'					=> '<var>default_charset</var> must be set to <samp>UTF-8</samp>. You can check the current value on the <samp>PHP information</samp> page.',
 
 	'FILES_PER_DAY'		=> 'Attachments per day',
 	'FORUM_STATS'		=> 'Board statistics',