diff --git a/includes/BootstrapHelperFunctions.php b/includes/BootstrapHelperFunctions.php
index c9c93e14de2..c8dc6f940b9 100644
--- a/includes/BootstrapHelperFunctions.php
+++ b/includes/BootstrapHelperFunctions.php
@@ -22,14 +22,20 @@
  *
  * @internal Only for use by Setup.php and Installer.
  * @since 1.38
- * @param string $installationPath The installation's base path, typically global $IP.
+ * @param string|null $installationPath The installation's base path,
+ *        as returned by wfDetectInstallPath().
+ *
  * @return string The path to the settings file
  */
-function wfDetectLocalSettingsFile( string $installationPath ): string {
+function wfDetectLocalSettingsFile( ?string $installationPath = null ): string {
 	if ( defined( 'MW_CONFIG_FILE' ) ) {
 		return MW_CONFIG_FILE;
 	}
 
+	if ( $installationPath === null ) {
+		$installationPath = wfDetectInstallPath();
+	}
+
 	// We could look for LocalSettings.yaml and LocalSettings.json,
 	// and use them if they exist. But having them in a web accessible
 	// place is dangerous, so better not to encourage that.
@@ -45,3 +51,28 @@ function wfDetectLocalSettingsFile( string $installationPath ): string {
 	define( 'MW_CONFIG_FILE', $configFile );
 	return $configFile;
 }
+
+/**
+ * Decide and remember where mediawiki is installed.
+ *
+ * This is used by Setup.php and will (if not already) store the result
+ * in the MW_INSTALL_PATH constant.
+ *
+ * The install path is detected based on the location of this file,
+ * but can be overwritten using the MW_INSTALL_PATH environment variable.
+ *
+ * @internal Only for use by Setup.php and Installer.
+ * @since 1.39
+ * @return string The path to the mediawiki installation
+ */
+function wfDetectInstallPath(): string {
+	if ( !defined( 'MW_INSTALL_PATH' ) ) {
+		$IP = getenv( 'MW_INSTALL_PATH' );
+		if ( $IP === false ) {
+			$IP = dirname( __DIR__ );
+		}
+		define( 'MW_INSTALL_PATH', $IP );
+	}
+
+	return MW_INSTALL_PATH;
+}
diff --git a/includes/Setup.php b/includes/Setup.php
index 562b1279805..8802569ac46 100644
--- a/includes/Setup.php
+++ b/includes/Setup.php
@@ -94,12 +94,13 @@ if ( !defined( 'MW_ENTRY_POINT' ) ) {
 	define( 'MW_ENTRY_POINT', 'unknown' );
 }
 
-if ( !defined( 'MW_INSTALL_PATH' ) ) {
-	define( 'MW_INSTALL_PATH', $IP );
-} else {
-	// enforce consistency
-	$IP = MW_INSTALL_PATH;
-}
+// The $IP variable is defined for use inside this file and by LocalSettings.php.
+// It is made available as a global variable for backwards compatibility,
+// but application logic should use the BaseDirectory config setting
+// or the MW_INSTALL_PATH constant.
+// The BaseDirectory setting is later defined to have the same value as MW_INSTALL_PATH.
+global $IP;
+$IP = $IP = wfDetectInstallPath();
 
 /**
  * Pre-config setup: Before loading LocalSettings.php
@@ -108,7 +109,6 @@ if ( !defined( 'MW_INSTALL_PATH' ) ) {
  */
 require_once "$IP/includes/AutoLoader.php";
 require_once "$IP/includes/Defines.php";
-require_once "$IP/includes/BootstrapHelperFunctions.php";
 
 // Load composer's autoloader if present
 if ( is_readable( "$IP/vendor/autoload.php" ) ) {
diff --git a/includes/WebStart.php b/includes/WebStart.php
index c2832404702..81871c8d1f5 100644
--- a/includes/WebStart.php
+++ b/includes/WebStart.php
@@ -47,12 +47,6 @@ header( 'X-Content-Type-Options: nosniff' );
 # its purpose.
 define( 'MEDIAWIKI', true );
 
-# Full path to the installation directory.
-$IP = getenv( 'MW_INSTALL_PATH' );
-if ( $IP === false ) {
-	$IP = dirname( __DIR__ );
-}
-
 /**
  * @param SettingsBuilder $settings
  *
@@ -66,7 +60,10 @@ function wfWebStartNoLocalSettings( SettingsBuilder $settings ) {
 	die();
 }
 
-require_once "$IP/includes/BootstrapHelperFunctions.php";
+require_once __DIR__ . "/BootstrapHelperFunctions.php";
+
+# Full path to the installation directory.
+$IP = wfDetectInstallPath();
 
 // If no LocalSettings file exists, try to display an error page
 // (use a callback because it depends on TemplateParser)
diff --git a/includes/installer/Installer.php b/includes/installer/Installer.php
index 115d9ac835f..db25ae4950c 100644
--- a/includes/installer/Installer.php
+++ b/includes/installer/Installer.php
@@ -646,7 +646,7 @@ abstract class Installer {
 	 * @return array|false
 	 */
 	public static function getExistingLocalSettings() {
-		global $IP;
+		$IP = wfDetectInstallPath();
 
 		// You might be wondering why this is here. Well if you don't do this
 		// then some poorly-formed extensions try to call their own classes
diff --git a/maintenance/doMaintenance.php b/maintenance/doMaintenance.php
index 66c21dfd57e..0398055eedb 100644
--- a/maintenance/doMaintenance.php
+++ b/maintenance/doMaintenance.php
@@ -49,8 +49,7 @@ if ( !$maintClass || !class_exists( $maintClass ) ) {
 // Define the MediaWiki entrypoint
 define( 'MEDIAWIKI', true );
 
-// This environment variable is ensured present by Maintenance.php.
-$IP = getenv( 'MW_INSTALL_PATH' );
+$IP = wfDetectInstallPath();
 
 // Get an object to start us off
 /** @var Maintenance $maintenance */
diff --git a/maintenance/includes/Maintenance.php b/maintenance/includes/Maintenance.php
index 6b6dbf7136e..4cb638bc4d0 100644
--- a/maintenance/includes/Maintenance.php
+++ b/maintenance/includes/Maintenance.php
@@ -1291,7 +1291,7 @@ abstract class Maintenance {
 	 * @return string
 	 */
 	public function loadSettings() {
-		global $wgCommandLineMode, $IP;
+		global $wgCommandLineMode;
 
 		if ( isset( $this->mOptions['conf'] ) ) {
 			// Define the constant instead of directly setting $settingsFile
@@ -1299,7 +1299,7 @@ abstract class Maintenance {
 			// MW_CONFIG_FILE if it is defined.
 			define( 'MW_CONFIG_FILE', $this->mOptions['conf'] );
 		}
-		$settingsFile = wfDetectLocalSettingsFile( $IP );
+		$settingsFile = wfDetectLocalSettingsFile();
 
 		if ( isset( $this->mOptions['wiki'] ) ) {
 			$wikiName = $this->mOptions['wiki'];
diff --git a/tests/phpunit/MediaWikiIntegrationTestCase.php b/tests/phpunit/MediaWikiIntegrationTestCase.php
index 8532badd9bc..cce1a4fa6ea 100644
--- a/tests/phpunit/MediaWikiIntegrationTestCase.php
+++ b/tests/phpunit/MediaWikiIntegrationTestCase.php
@@ -186,8 +186,7 @@ abstract class MediaWikiIntegrationTestCase extends PHPUnit\Framework\TestCase {
 	 * @beforeClass
 	 */
 	final public static function mediaWikiSetUpBeforeClass(): void {
-		global $IP;
-		$settingsFile = wfDetectLocalSettingsFile( $IP );
+		$settingsFile = wfDetectLocalSettingsFile();
 		if ( !is_file( $settingsFile ) ) {
 				echo "The file $settingsFile could not be found. "
 				. "Test case " . static::class . " extends " . self::class . " "
diff --git a/tests/phpunit/bootstrap.php b/tests/phpunit/bootstrap.php
index 98285c89a97..e38527a83d8 100644
--- a/tests/phpunit/bootstrap.php
+++ b/tests/phpunit/bootstrap.php
@@ -38,8 +38,9 @@ require_once "$IP/tests/common/TestSetup.php";
 
 // We don't use a settings file here but some code still assumes that one exists
 TestSetup::requireOnceInGlobalScope( "$IP/includes/BootstrapHelperFunctions.php" );
+
+$IP = wfDetectInstallPath(); // ensure MW_INSTALL_PATH is defined
 wfDetectLocalSettingsFile( $IP );
-define( 'MW_INSTALL_PATH', $IP );
 
 // these variables must be defined before setup runs
 $GLOBALS['IP'] = $IP;
diff --git a/tests/phpunit/phpunit.php b/tests/phpunit/phpunit.php
index 633af45c98e..059a1a2b1de 100644
--- a/tests/phpunit/phpunit.php
+++ b/tests/phpunit/phpunit.php
@@ -133,6 +133,8 @@ $wrapper = new PHPUnitMaintClass();
 $wrapper->setup();
 
 require_once "$IP/includes/BootstrapHelperFunctions.php";
+
+$IP = wfDetectInstallPath(); // ensure MW_INSTALL_PATH is defined
 wfDetectLocalSettingsFile( $IP );
 
 function wfPHPUnitSetup() {