diff --git a/phpBB/common.php b/phpBB/common.php
index 4ce3ba8a640..616be921181 100644
--- a/phpBB/common.php
+++ b/phpBB/common.php
@@ -51,20 +51,10 @@
 		$server_port = 443;
 	}
 
-	$script_name = (!empty($_SERVER['REQUEST_URI'])) ? $_SERVER['REQUEST_URI'] : getenv('REQUEST_URI');
-	if (!$script_name)
-	{
-		$script_name = (!empty($_SERVER['PHP_SELF'])) ? $_SERVER['PHP_SELF'] : getenv('PHP_SELF');
-	}
-
-	// $phpbb_root_path accounts for redirects from e.g. /adm
-	$script_path = trim(dirname($script_name)) . '/' . $phpbb_root_path . 'install/app.' . $phpEx;
-	// Replace any number of consecutive backslashes and/or slashes with a single slash
-	// (could happen on some proxy setups and/or Windows servers)
-	$script_path = preg_replace('#[\\\\/]{2,}#', '/', $script_path);
+	$script_path = phpbb_get_install_redirect($phpbb_root_path, $phpEx);
 
 	// Eliminate . and .. from the path
-	require($phpbb_root_path . 'phpbb/filesystem.' . $phpEx);
+	require($phpbb_root_path . 'phpbb/filesystem/filesystem.' . $phpEx);
 	$phpbb_filesystem = new phpbb\filesystem\filesystem();
 	$script_path = $phpbb_filesystem->clean_path($script_path);
 
diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index 7353fa2132e..17b3eda34e4 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -1814,6 +1814,31 @@ function redirect($url, $return = false, $disable_cd_check = false)
 	exit;
 }
 
+/**
+ * Returns the install redirect path for phpBB.
+ *
+ * @param string $phpbb_root_path The root path of the phpBB installation.
+ * @param string $phpEx The file extension of php files, e.g., "php".
+ * @return string The install redirect path.
+ */
+function phpbb_get_install_redirect(string $phpbb_root_path, string $phpEx): string
+{
+	$script_name = (!empty($_SERVER['REQUEST_URI'])) ? $_SERVER['REQUEST_URI'] : getenv('REQUEST_URI');
+	if (!$script_name)
+	{
+		$script_name = (!empty($_SERVER['PHP_SELF'])) ? $_SERVER['PHP_SELF'] : getenv('PHP_SELF');
+	}
+
+	// Add trailing dot to prevent dirname() from returning parent directory if $script_name is a directory
+	$script_name = substr($script_name, -1) === '/' ? $script_name . '.' : $script_name;
+
+	// $phpbb_root_path accounts for redirects from e.g. /adm
+	$script_path = trim(dirname($script_name)) . '/' . $phpbb_root_path . 'install/app.' . $phpEx;
+	// Replace any number of consecutive backslashes and/or slashes with a single slash
+	// (could happen on some proxy setups and/or Windows servers)
+	return preg_replace('#[\\\\/]{2,}#', '/', $script_path);
+}
+
 /**
 * Re-Apply session id after page reloads
 */
diff --git a/tests/functions/phpbb_get_install_redirect_test.php b/tests/functions/phpbb_get_install_redirect_test.php
new file mode 100644
index 00000000000..22b9ade5895
--- /dev/null
+++ b/tests/functions/phpbb_get_install_redirect_test.php
@@ -0,0 +1,68 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+class phpbb_get_install_redirect_test extends phpbb_test_case
+{
+	public function data_redirect(): array
+	{
+		return [
+			[
+				['REQUEST_URI'	=> '/foo/bar/'],
+				'/foo/bar/install/app.php',
+			],
+			[
+				['REQUEST_URI'	=> '/foo/bar/index.php'],
+				'/foo/bar/install/app.php',
+			],
+			[
+				['REQUEST_URI'	=> '/foo/bar'],
+				'/foo/install/app.php',
+			],
+			[
+				['REQUEST_URI'	=> '/foo/'],
+				'/foo/install/app.php',
+			],
+			[
+				['REQUEST_URI'	=> '/foo/index.php'],
+				'/foo/install/app.php',
+			],
+			[
+				[
+					'REQUEST_URI'	=> '/foo/bar/',
+					'PHP_SELF'		=> '/foo/bar/index.php'
+				],
+				'/foo/bar/install/app.php',
+			],
+			[
+				[
+					'REQUEST_URI'	=> '',
+					'PHP_SELF'		=> '/foo/bar/index.php'
+				],
+				'/foo/bar/install/app.php',
+			],
+		];
+	}
+
+	/**
+	 * @backupGlobals enabled
+	 * @dataProvider data_redirect
+	 */
+	public function test_install_redirect($server_vars, $expected)
+	{
+		$phpbb_root_path = '/';
+		$phpEx = 'php';
+
+		$_SERVER = array_merge($_SERVER, $server_vars);
+		$this->assertEquals($expected, phpbb_get_install_redirect($phpbb_root_path, $phpEx));
+	}
+}