diff --git a/tests/cases/README.txt b/tests/cases/README.txt
index 29514b24d..8cd917784 100644
--- a/tests/cases/README.txt
+++ b/tests/cases/README.txt
@@ -1,4 +1,10 @@
-The HTML files were saved from:
+To update the expected outputs, run the PHP tests with the environment variable
+"DISCUSSIONTOOLS_OVERWRITE_TESTS" set to any value, e.g.:
+
+     DISCUSSIONTOOLS_OVERWRITE_TESTS=1 php tests/phpunit/phpunit.php extensions/DiscussionTools/tests/phpunit/
+
+
+The input HTML files were saved from:
 
 pl-big:
 https://pl.wikipedia.org/w/index.php?action=render&title=Wikipedia:Kawiarenka/Artyku%C5%82y_dyskusja/Archiwum/2018-pa%C5%BAdziernik&oldid=55171451
diff --git a/tests/phpunit/CommentModifierTest.php b/tests/phpunit/CommentModifierTest.php
index 99f50d519..b15521a8c 100644
--- a/tests/phpunit/CommentModifierTest.php
+++ b/tests/phpunit/CommentModifierTest.php
@@ -43,8 +43,10 @@ class CommentModifierTest extends CommentTestCase {
 
 		$expectedDoc = self::createDocument( $expected );
 
-		// Uncomment this to write updated content to the "modified HTML" files:
-		// self::overwriteHtmlFile( $expectedPath, $doc, $origPath );
+		// Optionally write updated content to the "modified HTML" files
+		if ( getenv( 'DISCUSSIONTOOLS_OVERWRITE_TESTS' ) ) {
+			self::overwriteHtmlFile( $expectedPath, $doc, $origPath );
+		}
 
 		// saveHtml is not dirty-diff safe, but for testing it is probably faster than DOMCompat::getOuterHTML
 		self::assertEquals( $expectedDoc->saveHtml(), $doc->saveHtml(), $name );
@@ -87,8 +89,10 @@ class CommentModifierTest extends CommentTestCase {
 
 		$expectedDoc = self::createDocument( $expected );
 
-		// Uncomment this to write updated content to the "reply HTML" files:
-		// self::overwriteHtmlFile( $expectedPath, $doc, $origPath );
+		// Optionally write updated content to the "reply HTML" files
+		if ( getenv( 'DISCUSSIONTOOLS_OVERWRITE_TESTS' ) ) {
+			self::overwriteHtmlFile( $expectedPath, $doc, $origPath );
+		}
 
 		// saveHtml is not dirty-diff safe, but for testing it is probably faster than DOMCompat::getOuterHTML
 		self::assertEquals( $expectedDoc->saveHtml(), $doc->saveHtml(), $name );
diff --git a/tests/phpunit/CommentParserTest.php b/tests/phpunit/CommentParserTest.php
index 836ed7557..8ad4a6af1 100644
--- a/tests/phpunit/CommentParserTest.php
+++ b/tests/phpunit/CommentParserTest.php
@@ -203,8 +203,10 @@ class CommentParserTest extends CommentTestCase {
 			$processedThreads[] = $thread;
 		}
 
-		// Uncomment this to write updated content to the JSON files:
-		// self::overwriteJsonFile( $expectedPath, $processedThreads );
+		// Optionally write updated content to the JSON files
+		if ( getenv( 'DISCUSSIONTOOLS_OVERWRITE_TESTS' ) ) {
+			self::overwriteJsonFile( $expectedPath, $processedThreads );
+		}
 
 		foreach ( $threads as $i => $thread ) {
 			self::assertEquals( $expected[$i], $processedThreads[$i], $name . ' section ' . $i );
diff --git a/tests/phpunit/DataTest.php b/tests/phpunit/DataTest.php
index 5af2a6508..eadda0f93 100644
--- a/tests/phpunit/DataTest.php
+++ b/tests/phpunit/DataTest.php
@@ -24,8 +24,10 @@ class DataTest extends CommentTestCase {
 
 		$data = Data::getLocalData( null, $conf, $langCode );
 
-		// Uncomment this to write updated content to the JSON files:
-		// self::overwriteJsonFile( $expectedPath, $data );
+		// Optionally write updated content to the JSON files
+		if ( getenv( 'DISCUSSIONTOOLS_OVERWRITE_TESTS' ) ) {
+			self::overwriteJsonFile( $expectedPath, $data );
+		}
 
 		self::assertEquals( $expectedData, $data );
 	}
diff --git a/tests/phpunit/ThreadItemTest.php b/tests/phpunit/ThreadItemTest.php
index db4bcdfe7..c2d53f0c2 100644
--- a/tests/phpunit/ThreadItemTest.php
+++ b/tests/phpunit/ThreadItemTest.php
@@ -74,8 +74,10 @@ class ThreadItemTest extends CommentTestCase {
 			$transcludedFrom[ $comment->getId() ] = $comment->getTranscludedFrom();
 		}
 
-		// Uncomment this to write updated content to the JSON files:
-		// self::overwriteJsonFile( $expectedPath, $transcludedFrom );
+		// Optionally write updated content to the JSON files
+		if ( getenv( 'DISCUSSIONTOOLS_OVERWRITE_TESTS' ) ) {
+			self::overwriteJsonFile( $expectedPath, $transcludedFrom );
+		}
 
 		self::assertEquals(
 			$expected,