diff --git a/tests/Aws/Tests/S3/Sync/AbstractSyncTest.php b/tests/Aws/Tests/S3/Sync/AbstractSyncTest.php
index e5913b320d..84a64e9e36 100644
--- a/tests/Aws/Tests/S3/Sync/AbstractSyncTest.php
+++ b/tests/Aws/Tests/S3/Sync/AbstractSyncTest.php
@@ -25,8 +25,8 @@ class AbstractSyncTest extends \Guzzle\Tests\GuzzleTestCase
 {
     public function setUp()
     {
-        if (in_array(PHP_VERSION_ID, array(50429, 50513, 50600))) {
-            $this->markTestSkipped('TODO: Remove once PHPUnit is tagged.');
+        if (!\can_mock_internal_classes()) {
+            $this->markTestSkipped('Cannot mock internal classes');
         }
     }
 
diff --git a/tests/Aws/Tests/S3/Sync/ChangedFilesIteratorTest.php b/tests/Aws/Tests/S3/Sync/ChangedFilesIteratorTest.php
index 7a737a170d..bdcaf027d8 100644
--- a/tests/Aws/Tests/S3/Sync/ChangedFilesIteratorTest.php
+++ b/tests/Aws/Tests/S3/Sync/ChangedFilesIteratorTest.php
@@ -26,8 +26,8 @@ class ChangedFilesIteratorTest extends \Guzzle\Tests\GuzzleTestCase
 {
     public function setUp()
     {
-        if (in_array(PHP_VERSION_ID, array(50429, 50513, 50600))) {
-            $this->markTestSkipped('TODO: Remove once PHPUnit is tagged.');
+        if (!\can_mock_internal_classes()) {
+            $this->markTestSkipped('Cannot mock internal classes');
         }
     }
 
diff --git a/tests/Aws/Tests/S3/Sync/DownloadSyncTest.php b/tests/Aws/Tests/S3/Sync/DownloadSyncTest.php
index 9deecf5ca3..c3454e873c 100644
--- a/tests/Aws/Tests/S3/Sync/DownloadSyncTest.php
+++ b/tests/Aws/Tests/S3/Sync/DownloadSyncTest.php
@@ -27,8 +27,8 @@ class DownloadSyncTest extends \Guzzle\Tests\GuzzleTestCase
 {
     public function setUp()
     {
-        if (in_array(PHP_VERSION_ID, array(50429, 50513, 50600))) {
-            $this->markTestSkipped('TODO: Remove once PHPUnit is tagged.');
+        if (!\can_mock_internal_classes()) {
+            $this->markTestSkipped('Cannot mock internal classes');
         }
     }
 
diff --git a/tests/Aws/Tests/S3/Sync/UploadSyncTest.php b/tests/Aws/Tests/S3/Sync/UploadSyncTest.php
index d7d41ddb10..9b17462db3 100644
--- a/tests/Aws/Tests/S3/Sync/UploadSyncTest.php
+++ b/tests/Aws/Tests/S3/Sync/UploadSyncTest.php
@@ -30,8 +30,8 @@ class UploadSyncTest extends \Guzzle\Tests\GuzzleTestCase
 
     public function setUp()
     {
-        if (in_array(PHP_VERSION_ID, array(50429, 50513, 50600))) {
-            $this->markTestSkipped('TODO: Remove once PHPUnit is tagged.');
+        if (!\can_mock_internal_classes()) {
+            $this->markTestSkipped('Cannot mock internal classes');
         }
 
         $this->tmpFile = null;
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 37caf710ff..b2865b69cf 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -68,3 +68,17 @@
 
 // Emit deprecation warnings
 Guzzle\Common\Version::$emitWarnings = true;
+
+function can_mock_internal_classes()
+{
+    switch (substr(PHP_VERSION, 0, 3)) {
+        case '5.3.':
+            return true;
+        case '5.4.':
+            return version_compare(PHP_VERSION, '5.4.30', '<');
+        case '5.5.':
+            return version_compare(PHP_VERSION, '5.5.14', '<');
+        default:
+            return false;
+    }
+}