diff --git a/appveyor/test_task.bat b/appveyor/test_task.bat
index 9604524608c7..04aad4769c4b 100644
--- a/appveyor/test_task.bat
+++ b/appveyor/test_task.bat
@@ -91,7 +91,7 @@ mkdir c:\tests_tmp
 set TEST_PHP_JUNIT=c:\junit.out.xml
 
 cd "%APPVEYOR_BUILD_FOLDER%"
-nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 -g FAIL,XFAIL,BORK,WARN,LEAK,SKIP --temp-source c:\tests_tmp --temp-target c:\tests_tmp -j2"
+nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 -g FAIL,XFAIL,BORK,WARN,LEAK,SKIP --temp-source c:\tests_tmp --temp-target c:\tests_tmp -j3"
 
 set EXIT_CODE=%errorlevel%
 
diff --git a/run-tests.php b/run-tests.php
index 9b6e85ab401b..5e0011086cba 100755
--- a/run-tests.php
+++ b/run-tests.php
@@ -1283,7 +1283,8 @@ function run_all_tests($test_files, $env, $redir_tested = null)
 	global $PHP_FAILED_TESTS, $workers, $workerID, $workerSock;
 
 	if ($workers !== null && !$workerID) {
-		run_all_tests_parallel($test_files, $env, $redir_tested);
+		if ($redir_tested) die("redir_tested set for coordinator process");
+		run_all_tests_parallel($test_files, $env);
 		return;
 	}
 
@@ -1336,7 +1337,7 @@ function run_all_tests($test_files, $env, $redir_tested = null)
 }
 
 /** The heart of parallel testing. */
-function run_all_tests_parallel($test_files, $env, $redir_tested) {
+function run_all_tests_parallel($test_files, $env) {
 	global $workers, $test_idx, $test_cnt, $test_results, $failed_tests_file, $result_tests_file, $PHP_FAILED_TESTS, $shuffle;
 
 	// The PHP binary running run-tests.php, and run-tests.php itself
@@ -1411,12 +1412,19 @@ function run_all_tests_parallel($test_files, $env, $redir_tested) {
 	$sockUri = "tcp://$sockHost:$sockPort";
 
 	for ($i = 1; $i <= $workers; $i++) {
+		$workerEnv = $env;
+		// Use separate TEMP dir on Windows to get separate opcache instances.
+		if (PHP_OS_FAMILY == 'Windows') {
+			$workerEnv['TEMP'] .= '\\run-tests-' . $i;
+			@mkdir($workerEnv['TEMP']);
+		}
+
 		$proc = proc_open(
 			$thisPHP . ' ' . escapeshellarg($thisScript),
 			[], // Inherit our stdin, stdout and stderr
 			$pipes,
 			NULL,
-			$_ENV + [
+			$workerEnv + [
 				"TEST_PHP_WORKER" => $i,
 				"TEST_PHP_URI" => $sockUri,
 			],
@@ -1560,8 +1568,6 @@ function run_all_tests_parallel($test_files, $env, $redir_tested) {
 								send_message($workerSocks[$i], [
 									"type" => "run_tests",
 									"test_files" => $files,
-									"env" => $env,
-									"redir_tested" => $redir_tested
 								]);
 							} else {
 								proc_terminate($workerProcs[$i]);
@@ -1672,7 +1678,7 @@ function run_worker() {
 	});
 
 	foreach ($greeting["GLOBALS"] as $var => $value) {
-		if ($var !== "workerID" && $var !== "workerSock" && $var !== "GLOBALS") {
+		if ($var !== "workerID" && $var !== "workerSock" && $var !== "GLOBALS" && $var !== "_ENV") {
 			$GLOBALS[$var] = $value;
 		}
 	}
@@ -1694,7 +1700,7 @@ function run_worker() {
 
 		switch ($command["type"]) {
 			case "run_tests":
-				run_all_tests($command["test_files"], $command["env"], $command["redir_tested"]);
+				run_all_tests($command["test_files"], $_ENV);
 				send_message($workerSock, [
 					"type" => "tests_finished"
 				]);