diff --git a/adminer/call.inc.php b/adminer/call.inc.php
index b9ffbaa5..aae0012a 100644
--- a/adminer/call.inc.php
+++ b/adminer/call.inc.php
@@ -42,7 +42,7 @@ if (!$error && $_POST) {
 		echo "<p class='error'>" . error() . "\n";
 	} else {
 		$connection2 = connect(adminer()->credentials());
-		if (is_object($connection2)) {
+		if ($connection2) {
 			$connection2->select_db(DB);
 		}
 
diff --git a/adminer/drivers/mysql.inc.php b/adminer/drivers/mysql.inc.php
index 9203a0ae..260d8ea3 100644
--- a/adminer/drivers/mysql.inc.php
+++ b/adminer/drivers/mysql.inc.php
@@ -8,7 +8,7 @@ if (!defined('Adminer\DRIVER')) {
 	// MySQLi supports everything, MySQL doesn't support multiple result sets, PDO_MySQL doesn't support orgtable
 	if (extension_loaded("mysqli") && $_GET["ext"] != "pdo") {
 		class Db extends \MySQLi {
-			/** @var Db|string */ static $instance = '';
+			/** @var Db */ static $instance;
 			public string $extension = "MySQLi", $flavor = '';
 
 			function __construct() {
@@ -533,10 +533,9 @@ if (!defined('Adminer\DRIVER')) {
 	}
 
 	/** Get table indexes
-	* @param Db|string $connection2
 	* @return Index[]
 	*/
-	function indexes(string $table, $connection2 = null): array {
+	function indexes(string $table, Db $connection2 = null): array {
 		$return = array();
 		foreach (get_rows("SHOW INDEX FROM " . table($table), $connection2) as $row) {
 			$name = $row["Key_name"];
@@ -1065,9 +1064,8 @@ if (!defined('Adminer\DRIVER')) {
 	}
 
 	/** Set current schema
-	* @param Db|string $connection2
 	*/
-	function set_schema(string $schema, $connection2 = null): bool {
+	function set_schema(string $schema, Db $connection2 = null): bool {
 		return true;
 	}
 }
diff --git a/adminer/include/auth.inc.php b/adminer/include/auth.inc.php
index d9a75a15..9a5f5cdd 100644
--- a/adminer/include/auth.inc.php
+++ b/adminer/include/auth.inc.php
@@ -167,24 +167,27 @@ if (isset($_GET["username"]) && !class_exists('Adminer\Db')) {
 	exit;
 }
 
+$connection = '';
 if (isset($_GET["username"]) && is_string(get_password())) {
 	list($host, $port) = explode(":", SERVER, 2);
 	if (preg_match('~^\s*([-+]?\d+)~', $port, $match) && ($match[1] < 1024 || $match[1] > 65535)) { // is_numeric('80#') would still connect to port 80
 		auth_error(lang('Connecting to privileged ports is not allowed.'), $permanent);
 	}
 	check_invalid_login($permanent);
-	Db::$instance = connect(adminer()->credentials());
-	if (is_object(Db::$instance)) {
-		Driver::$instance = new Driver(Db::$instance);
-		if (Db::$instance->flavor) {
+	$credentials = adminer()->credentials();
+	$connection = Driver::connect($credentials[0], $credentials[1], $credentials[2]);
+	if (is_object($connection)) {
+		Db::$instance = $connection;
+		Driver::$instance = new Driver($connection);
+		if ($connection->flavor) {
 			save_settings(array("vendor-" . DRIVER . "-" . SERVER => get_driver(DRIVER)));
 		}
 	}
 }
 
 $login = null;
-if (!is_object(connection()) || ($login = adminer()->login($_GET["username"], get_password())) !== true) {
-	$error = (is_string(connection()) ? nl_br(h(connection())) : (is_string($login) ? $login : lang('Invalid credentials.')));
+if (!is_object($connection) || ($login = adminer()->login($_GET["username"], get_password())) !== true) {
+	$error = (is_string($connection) ? nl_br(h($connection)) : (is_string($login) ? $login : lang('Invalid credentials.')));
 	auth_error(
 		$error . (preg_match('~^ | $~', get_password()) ? '<br>' . lang('There is a space in the input password which might be the cause.') : ''),
 		$permanent
diff --git a/adminer/include/db.inc.php b/adminer/include/db.inc.php
index 2477231f..2260a2f5 100644
--- a/adminer/include/db.inc.php
+++ b/adminer/include/db.inc.php
@@ -4,7 +4,7 @@ namespace Adminer;
 // this could be interface when "Db extends \mysqli" can have compatible type declarations (PHP 7)
 // interfaces can include properties only since PHP 8.4
 abstract class SqlDb {
-	/** @var Db|string */ static $instance = ''; // string means error
+	/** @var Db */ static $instance;
 
 	public string $extension; // extension name
 	public string $flavor = ''; // different vendor with the same API, e.g. MariaDB; usually stays empty
diff --git a/adminer/include/editing.inc.php b/adminer/include/editing.inc.php
index 68c4116e..026e5561 100644
--- a/adminer/include/editing.inc.php
+++ b/adminer/include/editing.inc.php
@@ -5,7 +5,6 @@ namespace Adminer;
 
 /** Print select result
 * @param Result $result
-* @param Db|string $connection2
 * @param string[] $orgtables
 * @param int|numeric-string $limit
 * @return string[] $orgtables
diff --git a/adminer/include/functions.inc.php b/adminer/include/functions.inc.php
index 29ac368a..cd033ccd 100644
--- a/adminer/include/functions.inc.php
+++ b/adminer/include/functions.inc.php
@@ -4,12 +4,12 @@ namespace Adminer;
 // This file is used both in Adminer and Adminer Editor.
 
 /** Get database connection
-* @param Db|string $connection2 custom connection to use instead of the default
-* @return Db|string string means error
+* @param ?Db $connection2 custom connection to use instead of the default
+* @return Db
 */
-function connection($connection2 = null) {
+function connection(Db $connection2 = null) {
 	// can be used in customization, Db::$instance is minified
-	return (is_object($connection2) ? $connection2 : Db::$instance);
+	return ($connection2 ?: Db::$instance);
 }
 
 /** Get Adminer object
@@ -26,10 +26,10 @@ function driver(): Driver {
 
 /** Connect to the database
 * @param array{?string, string, string} $credentials [$server, $username, $password]
-* @return Db|string string for error
 */
-function connect(array $credentials) {
-	return driver()->connect($credentials[0], $credentials[1], $credentials[2]);
+function connect(array $credentials): ?Db {
+	$return = driver()->connect($credentials[0], $credentials[1], $credentials[2]);
+	return (is_object($return) ? $return : null);
 }
 
 /** Unescape database identifier
@@ -105,9 +105,8 @@ function bracket_escape(string $idf, bool $back = false): string {
 /** Check if connection has at least the given version
 * @param string|float $version required version
 * @param string|float $maria_db required MariaDB version
-* @param Db|string $connection2 defaults to connection()
 */
-function min_version($version, $maria_db = "", $connection2 = null): bool {
+function min_version($version, $maria_db = "", Db $connection2 = null): bool {
 	$connection2 = connection($connection2);
 	$server_info = $connection2->server_info;
 	if ($maria_db && preg_match('~([\d.]+)-MariaDB~', $server_info, $match)) {
@@ -188,10 +187,9 @@ function get_vals(string $query, $column = 0): array {
 }
 
 /** Get keys from first column and values from second
-* @param Db|string $connection2
 * @return string[]
 */
-function get_key_vals(string $query, $connection2 = null, bool $set_keys = true): array {
+function get_key_vals(string $query, Db $connection2 = null, bool $set_keys = true): array {
 	$connection2 = connection($connection2);
 	$return = array();
 	$result = $connection2->query($query);
@@ -208,10 +206,9 @@ function get_key_vals(string $query, $connection2 = null, bool $set_keys = true)
 }
 
 /** Get all rows of result
-* @param Db|string $connection2
 * @return list<string[]> of associative arrays
 */
-function get_rows(string $query, $connection2 = null, string $error = "<p class='error'>"): array {
+function get_rows(string $query, Db $connection2 = null, string $error = "<p class='error'>"): array {
 	$conn = connection($connection2);
 	$return = array();
 	$result = $conn->query($query);
@@ -219,7 +216,7 @@ function get_rows(string $query, $connection2 = null, string $error = "<p class=
 		while ($row = $result->fetch_assoc()) {
 			$return[] = $row;
 		}
-	} elseif (!$result && $connection2 === null && $error && (defined('Adminer\PAGE_HEADER') || $error == "-- ")) {
+	} elseif (!$result && !$connection2 && $error && (defined('Adminer\PAGE_HEADER') || $error == "-- ")) {
 		echo $error . error() . "\n";
 	}
 	return $return;
@@ -833,7 +830,7 @@ function slow_query(string $query): array {
 	$connection2 = null;
 	if (!$slow_query && support("kill")) {
 		$connection2 = connect(adminer()->credentials());
-		if (is_object($connection2) && ($db == "" || $connection2->select_db($db))) {
+		if ($connection2 && ($db == "" || $connection2->select_db($db))) {
 			$kill = get_val(connection_id(), 0, $connection2); // MySQL and MySQLi can use thread_id but it's not in PDO_MySQL
 			echo script("const timeout = setTimeout(() => { ajax('" . js_escape(ME) . "script=kill', function () {}, 'kill=$kill&token=" . get_token() . "'); }, 1000 * $timeout);");
 		}
diff --git a/adminer/sql.inc.php b/adminer/sql.inc.php
index 4860c5fb..ebb354e5 100644
--- a/adminer/sql.inc.php
+++ b/adminer/sql.inc.php
@@ -55,7 +55,7 @@ if (!$error && $_POST) {
 		$offset = 0;
 		$empty = true;
 		$connection2 = connect(adminer()->credentials()); // connection for exploring indexes and EXPLAIN (to not replace FOUND_ROWS()) //! PDO - silent error
-		if (is_object($connection2) && DB != "") {
+		if ($connection2 && DB != "") {
 			$connection2->select_db(DB);
 			if ($_GET["ns"] != "") {
 				set_schema($_GET["ns"], $connection2);
@@ -126,7 +126,7 @@ if (!$error && $_POST) {
 							}
 							$start = microtime(true);
 							//! don't allow changing of character_set_results, convert encoding of displayed query
-							if (connection()->multi_query($q) && is_object($connection2) && preg_match("~^$space*+USE\\b~i", $q)) {
+							if (connection()->multi_query($q) && $connection2 && preg_match("~^$space*+USE\\b~i", $q)) {
 								$connection2->query($q);
 							}
 
diff --git a/compile.php b/compile.php
index e248013e..60d0056d 100755
--- a/compile.php
+++ b/compile.php
@@ -278,7 +278,6 @@ if ($_SERVER["argv"][1]) {
 include __DIR__ . "/adminer/include/db.inc.php";
 include __DIR__ . "/adminer/include/pdo.inc.php";
 include __DIR__ . "/adminer/include/driver.inc.php";
-Adminer\SqlDb::$instance = (object) array('flavor' => '', 'server_info' => '99'); // used in support()
 $features = array("check", "call" => "routine", "dump", "event", "privileges", "procedure" => "routine", "processlist", "routine", "scheme", "sequence", "sql", "status", "trigger", "type", "user" => "privileges", "variables", "view");
 $lang_ids = array(); // global variable simplifies usage in a callback function
 $file = file_get_contents(__DIR__ . "/$project/index.php");
@@ -286,6 +285,7 @@ $file = preg_replace('~\*/~', "* @version " . Adminer\VERSION . "\n*/", $file, 1
 if ($vendor) {
 	$_GET[$vendor] = true; // to load the driver
 	include_once __DIR__ . $driver_path;
+	Adminer\Db::$instance = (object) array('flavor' => '', 'server_info' => '99'); // used in support()
 	foreach ($features as $key => $feature) {
 		if (!Adminer\support($feature)) {
 			if (!is_int($key)) {