diff --git a/includes/GlobalFunctions.php b/includes/GlobalFunctions.php
index 7f0c2bf36ab..a396af6d40c 100644
--- a/includes/GlobalFunctions.php
+++ b/includes/GlobalFunctions.php
@@ -1841,11 +1841,7 @@ function wfTimestampNow() {
  * @return bool True if it's Windows, false otherwise.
  */
 function wfIsWindows() {
-	static $isWindows = null;
-	if ( $isWindows === null ) {
-		$isWindows = strtoupper( substr( PHP_OS, 0, 3 ) ) === 'WIN';
-	}
-	return $isWindows;
+	return PHP_OS_FAMILY === 'Windows';
 }
 
 /**
diff --git a/includes/libs/filebackend/FSFileBackend.php b/includes/libs/filebackend/FSFileBackend.php
index 581b577f591..612ad4e35d3 100644
--- a/includes/libs/filebackend/FSFileBackend.php
+++ b/includes/libs/filebackend/FSFileBackend.php
@@ -77,7 +77,7 @@ class FSFileBackend extends FileBackendStore {
 	/** @var string Required OS username to own files */
 	protected $fileOwner;
 
-	/** @var bool Simpler version of PHP_OS_FAMILY (PHP 7.2) */
+	/** @var bool Simpler version of PHP_OS_FAMILY */
 	protected $os;
 	/** @var string OS username running this script */
 	protected $currentUser;
@@ -98,9 +98,9 @@ class FSFileBackend extends FileBackendStore {
 	public function __construct( array $config ) {
 		parent::__construct( $config );
 
-		if ( substr( PHP_OS, 0, 3 ) === 'WIN' || PHP_OS === 'Windows' ) {
+		if ( PHP_OS_FAMILY === 'Windows' ) {
 			$this->os = 'Windows';
-		} elseif ( substr( PHP_OS, 0, -3 ) === 'BSD' || PHP_OS === 'Darwin' ) {
+		} elseif ( PHP_OS_FAMILY === 'BSD' || PHP_OS_FAMILY === 'Darwin' ) {
 			$this->os = 'BSD';
 		} else {
 			$this->os = 'Linux';
diff --git a/includes/libs/filebackend/fsfile/TempFSFile.php b/includes/libs/filebackend/fsfile/TempFSFile.php
index 6f87438aea6..4d6f620c522 100644
--- a/includes/libs/filebackend/fsfile/TempFSFile.php
+++ b/includes/libs/filebackend/fsfile/TempFSFile.php
@@ -92,7 +92,7 @@ class TempFSFile extends FSFile {
 		// the current process.
 		// The user is included as if various scripts are run by different users they will likely
 		// not be able to access each others temporary files.
-		if ( strtoupper( substr( PHP_OS, 0, 3 ) ) === 'WIN' ) {
+		if ( PHP_OS_FAMILY === 'Windows' ) {
 			$tmp = sys_get_temp_dir() . DIRECTORY_SEPARATOR . 'mwtmp-' . get_current_user();
 			if ( !is_dir( $tmp ) ) {
 				mkdir( $tmp );
diff --git a/includes/libs/lockmanager/FSLockManager.php b/includes/libs/lockmanager/FSLockManager.php
index 86336d38cae..a901e1049db 100644
--- a/includes/libs/lockmanager/FSLockManager.php
+++ b/includes/libs/lockmanager/FSLockManager.php
@@ -60,7 +60,7 @@ class FSLockManager extends LockManager {
 		parent::__construct( $config );
 
 		$this->lockDir = $config['lockDirectory'];
-		$this->isWindows = ( strtoupper( substr( PHP_OS, 0, 3 ) ) === 'WIN' );
+		$this->isWindows = ( PHP_OS_FAMILY === 'Windows' );
 	}
 
 	/**
diff --git a/includes/libs/uuid/GlobalIdGenerator.php b/includes/libs/uuid/GlobalIdGenerator.php
index 428d15b196f..a8a74cf7d40 100644
--- a/includes/libs/uuid/GlobalIdGenerator.php
+++ b/includes/libs/uuid/GlobalIdGenerator.php
@@ -645,7 +645,7 @@ class GlobalIdGenerator {
 		// Try to get some ID that uniquely identifies this machine (RFC 4122)...
 		if ( !preg_match( '/^[0-9a-f]{12}$/i', $nodeId ) ) {
 			AtEase::suppressWarnings();
-			if ( strtoupper( substr( PHP_OS, 0, 3 ) ) === 'WIN' ) {
+			if ( PHP_OS_FAMILY === 'Windows' ) {
 				// https://technet.microsoft.com/en-us/library/bb490913.aspx
 				$csv = trim( ( $this->shellCallback )( 'getmac /NH /FO CSV' ) );
 				$line = substr( $csv, 0, strcspn( $csv, "\n" ) );