diff --git a/lib/php/README.md b/lib/php/README.md
index c24ee2c..7170104 100644
--- a/lib/php/README.md
+++ b/lib/php/README.md
@@ -1,7 +1,6 @@
 Thrift PHP Software Library
 
-License
-=======
+# License
 
 Licensed to the Apache Software Foundation (ASF) under one
 or more contributor license agreements. See the NOTICE file
@@ -20,8 +19,7 @@
 specific language governing permissions and limitations
 under the License.
 
-Using Thrift with PHP
-=====================
+# Using Thrift with PHP
 
 Thrift requires PHP 5. Thrift makes as few assumptions about your PHP
 environment as possible while trying to make some more advanced PHP
@@ -29,25 +27,34 @@
 
 To use Thrift in your PHP codebase, take the following steps:
 
-#1) Copy all of thrift/lib/php/lib into your PHP codebase
-#2) Configure Symfony Autoloader (or whatever you usually use)
+1. Copy all of thrift/lib/php/lib into your PHP codebase
+2. Configure Symfony Autoloader (or whatever you usually use)
 
 After that, you have to manually include the Thrift package
 created by the compiler:
 
+```
 require_once 'packages/Service/Service.php';
 require_once 'packages/Service/Types.php';
+```
 
-Dependencies
-============
+# Dependencies
 
 PHP_INT_SIZE
 
-  This built-in signals whether your architecture is 32 or 64 bit and is
-  used by the TBinaryProtocol to properly use pack() and unpack() to
-  serialize data.
+    This built-in signals whether your architecture is 32 or 64 bit and is
+    used by the TBinaryProtocol to properly use pack() and unpack() to
+    serialize data.
 
 apc_fetch(), apc_store()
 
-  APC cache is used by the TSocketPool class. If you do not have APC installed,
-  Thrift will fill in null stub function definitions.
+    APC cache is used by the TSocketPool class. If you do not have APC installed,
+    Thrift will fill in null stub function definitions.
+
+# Breaking Changes
+
+## 0.12.0
+
+1. [PSR-4](https://www.php-fig.org/psr/psr-4/) loader is now the default. If you want to use class maps instead, use `-gen php:classmap`.
+
+2. If using PSR-4, use `$thriftClassLoader->registerNamespace('namespace', '<path>')` instead of `$thriftClassLoader->registerDefinition('namespace', '<path>')`.
diff --git a/lib/php/test/JsonSerialize/JsonSerializeTest.php b/lib/php/test/JsonSerialize/JsonSerializeTest.php
index 8c64595..c668652 100644
--- a/lib/php/test/JsonSerialize/JsonSerializeTest.php
+++ b/lib/php/test/JsonSerialize/JsonSerializeTest.php
@@ -22,9 +22,8 @@
 
 use PHPUnit\Framework\TestCase;
 use stdClass;
-use Thrift\ClassLoader\ThriftClassLoader;
 
-require_once __DIR__ . '/../../../../vendor/autoload.php';
+require __DIR__ . '/../../../../vendor/autoload.php';
 
 /**
  * @runTestsInSeparateProcesses
@@ -36,9 +35,9 @@
         if (version_compare(phpversion(), '5.4', '<')) {
             $this->markTestSkipped('Requires PHP 5.4 or newer!');
         }
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages/phpjs');
-        $loader->register();
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/phpjs');
     }
 
     public function testEmptyStruct()
diff --git a/lib/php/test/Makefile.am b/lib/php/test/Makefile.am
index 5c86e35..4824688 100755
--- a/lib/php/test/Makefile.am
+++ b/lib/php/test/Makefile.am
@@ -20,8 +20,8 @@
 PHPUNIT=php $(top_srcdir)/vendor/bin/phpunit
 
 stubs: ../../../test/ThriftTest.thrift  TestValidators.thrift
-	mkdir -p ./packages
-	$(THRIFT) --gen php -r --out ./packages ../../../test/ThriftTest.thrift
+	mkdir -p ./packages/php
+	$(THRIFT) --gen php -r --out ./packages/php ../../../test/ThriftTest.thrift
 	mkdir -p ./packages/phpv
 	mkdir -p ./packages/phpvo
 	mkdir -p ./packages/phpjs
diff --git a/lib/php/test/Protocol/BinarySerializerTest.php b/lib/php/test/Protocol/BinarySerializerTest.php
index 7e87280..71b0bb5 100644
--- a/lib/php/test/Protocol/BinarySerializerTest.php
+++ b/lib/php/test/Protocol/BinarySerializerTest.php
@@ -24,10 +24,9 @@
 namespace Test\Thrift\Protocol;
 
 use PHPUnit\Framework\TestCase;
-use Thrift\ClassLoader\ThriftClassLoader;
 use Thrift\Serializer\TBinarySerializer;
 
-require_once __DIR__ . '/../../../../vendor/autoload.php';
+require __DIR__ . '/../../../../vendor/autoload.php';
 
 /***
  * This test suite depends on running the compiler against the
@@ -42,9 +41,9 @@
 {
     public function setUp()
     {
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages');
-        $loader->register();
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/php');
     }
 
     /**
diff --git a/lib/php/test/Protocol/TJSONProtocolTest.php b/lib/php/test/Protocol/TJSONProtocolTest.php
index bab4389..bf0ecce 100644
--- a/lib/php/test/Protocol/TJSONProtocolTest.php
+++ b/lib/php/test/Protocol/TJSONProtocolTest.php
@@ -25,11 +25,10 @@
 
 use PHPUnit\Framework\TestCase;
 use Test\Thrift\Fixtures;
-use Thrift\ClassLoader\ThriftClassLoader;
 use Thrift\Protocol\TJSONProtocol;
 use Thrift\Transport\TMemoryBuffer;
 
-require_once __DIR__ . '/../../../../vendor/autoload.php';
+require __DIR__ . '/../../../../vendor/autoload.php';
 
 /***
  * This test suite depends on running the compiler against the
@@ -47,9 +46,9 @@
 
     public static function setUpBeforeClass()
     {
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages');
-        $loader->register();
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/php');
 
         Fixtures::populateTestArgs();
         TJSONProtocolFixtures::populateTestArgsJSON();
diff --git a/lib/php/test/Protocol/TSimpleJSONProtocolTest.php b/lib/php/test/Protocol/TSimpleJSONProtocolTest.php
index ec64321..e4a1373 100644
--- a/lib/php/test/Protocol/TSimpleJSONProtocolTest.php
+++ b/lib/php/test/Protocol/TSimpleJSONProtocolTest.php
@@ -25,11 +25,10 @@
 
 use PHPUnit\Framework\TestCase;
 use Test\Thrift\Fixtures;
-use Thrift\ClassLoader\ThriftClassLoader;
 use Thrift\Protocol\TSimpleJSONProtocol;
 use Thrift\Transport\TMemoryBuffer;
 
-require_once __DIR__ . '/../../../../vendor/autoload.php';
+require __DIR__ . '/../../../../vendor/autoload.php';
 
 /***
  * This test suite depends on running the compiler against the
@@ -47,9 +46,10 @@
 
     public static function setUpBeforeClass()
     {
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages');
-        $loader->register();
+
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/php');
 
         Fixtures::populateTestArgs();
         TSimpleJSONProtocolFixtures::populateTestArgsSimpleJSON();
diff --git a/lib/php/test/Validator/ValidatorTest.php b/lib/php/test/Validator/ValidatorTest.php
index ace3075..fa6c7a9 100644
--- a/lib/php/test/Validator/ValidatorTest.php
+++ b/lib/php/test/Validator/ValidatorTest.php
@@ -20,7 +20,7 @@
 
 namespace Test\Thrift;
 
-require_once __DIR__ . '/../../../../vendor/autoload.php';
+require __DIR__ . '/../../../../vendor/autoload.php';
 
 use Thrift\ClassLoader\ThriftClassLoader;
 
@@ -34,9 +34,8 @@
 {
     public function setUp()
     {
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages/phpv');
-        $loader->registerDefinition('TestValidators', __DIR__ . '/../packages/phpv');
-        $loader->register();
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/phpv');
     }
 }
diff --git a/lib/php/test/Validator/ValidatorTestOop.php b/lib/php/test/Validator/ValidatorTestOop.php
index 5a8e3bf..93bca4d 100644
--- a/lib/php/test/Validator/ValidatorTestOop.php
+++ b/lib/php/test/Validator/ValidatorTestOop.php
@@ -34,9 +34,8 @@
 {
     public function setUp()
     {
-        $loader = new ThriftClassLoader();
-        $loader->registerDefinition('ThriftTest', __DIR__ . '/../packages/phpvo');
-        $loader->registerDefinition('TestValidators', __DIR__ . '/../packages/phpvo');
-        $loader->register();
+        /** @var \Composer\Autoload\ClassLoader $loader */
+        $loader = require __DIR__ . '/../../../../vendor/autoload.php';
+        $loader->addPsr4('', __DIR__ . '/../packages/phpvo');
     }
 }