diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index b6d942c51..828b98e75 100644
--- a/src/Composer/Config.php
+++ b/src/Composer/Config.php
@@ -44,6 +44,7 @@ class Config
         'github-domains' => array('github.com'),
         'github-expose-hostname' => true,
         'store-auths' => 'prompt',
+        'platform' => array(),
         // valid keys without defaults (auth config stuff):
         // github-oauth
         // http-basic
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index 7333d0d10..5934dd0df 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -206,7 +206,9 @@ class Installer
 
         // create installed repo, this contains all local packages + platform packages (php & extensions)
         $localRepo = $this->repositoryManager->getLocalRepository();
-        $platformRepo = new PlatformRepository();
+        $platformOverride = $this->config->get('platform');
+        $platformOverride = is_array($platformOverride) ? $platformOverride : array();
+        $platformRepo = new PlatformRepository($platformOverride);
         $repos = array(
             $localRepo,
             new InstalledArrayRepository(array($installedRootPackage)),
diff --git a/src/Composer/Repository/PlatformRepository.php b/src/Composer/Repository/PlatformRepository.php
index 636cba16b..909eaab7c 100644
--- a/src/Composer/Repository/PlatformRepository.php
+++ b/src/Composer/Repository/PlatformRepository.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Repository;
 
+use Composer\Package\PackageInterface;
 use Composer\Package\CompletePackage;
 use Composer\Package\Version\VersionParser;
 use Composer\Plugin\PluginInterface;
@@ -23,17 +24,41 @@ class PlatformRepository extends ArrayRepository
 {
     const PLATFORM_PACKAGE_REGEX = '{^(?:php(?:-64bit)?|hhvm|(?:ext|lib)-[^/]+)$}i';
 
+    private $overrides;
+
+    public function __construct(array $overrides = array())
+    {
+        parent::__construct(array());
+        $this->overrides = $overrides;
+    }
+
     protected function initialize()
     {
         parent::initialize();
 
         $versionParser = new VersionParser();
 
+        // Add each of the override versions as options.
+        // Later we might even replace the extensions instead.
+        foreach( $this->overrides as $name => $prettyVersion ) {
+            // Check that it's a platform package.
+            if( preg_match(self::PLATFORM_PACKAGE_REGEX, $name) ) {
+                $version = $versionParser->normalize($prettyVersion);
+                $package = new CompletePackage($name, $version, $prettyVersion);
+                $package->setDescription("Overridden virtual platform package $name.");
+                parent::addPackage($package);
+            }
+            else {
+                throw new \InvalidArgumentException('Invalid platform package '.$name);
+            }
+        }
+
+
         $prettyVersion = PluginInterface::PLUGIN_API_VERSION;
         $version = $versionParser->normalize($prettyVersion);
         $composerPluginApi = new CompletePackage('composer-plugin-api', $version, $prettyVersion);
         $composerPluginApi->setDescription('The Composer Plugin API');
-        parent::addPackage($composerPluginApi);
+        $this->addPackage($composerPluginApi);
 
         try {
             $prettyVersion = PHP_VERSION;
@@ -45,12 +70,12 @@ class PlatformRepository extends ArrayRepository
 
         $php = new CompletePackage('php', $version, $prettyVersion);
         $php->setDescription('The PHP interpreter');
-        parent::addPackage($php);
+        $this->addPackage($php);
 
         if (PHP_INT_SIZE === 8) {
             $php64 = new CompletePackage('php-64bit', $version, $prettyVersion);
             $php64->setDescription('The PHP interpreter (64bit)');
-            parent::addPackage($php64);
+            $this->addPackage($php64);
         }
 
         $loadedExtensions = get_loaded_extensions();
@@ -73,7 +98,7 @@ class PlatformRepository extends ArrayRepository
             $packageName = $this->buildPackageName($name);
             $ext = new CompletePackage($packageName, $version, $prettyVersion);
             $ext->setDescription('The '.$name.' PHP extension');
-            parent::addPackage($ext);
+            $this->addPackage($ext);
         }
 
         // Another quick loop, just for possible libraries
@@ -143,7 +168,7 @@ class PlatformRepository extends ArrayRepository
 
             $lib = new CompletePackage('lib-'.$name, $version, $prettyVersion);
             $lib->setDescription('The '.$name.' PHP library');
-            parent::addPackage($lib);
+            $this->addPackage($lib);
         }
 
         if (defined('HHVM_VERSION')) {
@@ -157,8 +182,28 @@ class PlatformRepository extends ArrayRepository
 
             $hhvm = new CompletePackage('hhvm', $version, $prettyVersion);
             $hhvm->setDescription('The HHVM Runtime (64bit)');
-            parent::addPackage($hhvm);
+            $this->addPackage($hhvm);
+        }
+    }
+
+    // TODO: Is it a good thing to redefine the public interface
+    // like this, or is it better to make the "only-add-if-no-in-platform"
+    // feature in a
+    // protected function addOverriddenPackage()
+    // instead?
+    public function addPackage(PackageInterface $package)
+    {
+        /*
+           If we can find the package in this repository,
+           in any version, it can only mean that it has been
+           added by the config key 'platform' and should
+           the real package (i.e. this one) should not be added.
+        */
+        if( count($this->findPackages($package->getName())) > 0 ) {
+            // Log a warning that we're ignoring existing package?
+            return;
         }
+        parent::addPackage($package);
     }
 
     private function buildPackageName($name)