diff --git a/phpBB/config/default/container/services_extensions.yml b/phpBB/config/default/container/services_extensions.yml
index 3a2e83f73a..46953ae4a2 100644
--- a/phpBB/config/default/container/services_extensions.yml
+++ b/phpBB/config/default/container/services_extensions.yml
@@ -18,9 +18,10 @@ services:
             - @config
 
     ext.composer.manager:
-        class: phpbb\composer\manager
+        class: phpbb\composer\extension_manager
         arguments:
             - @ext.composer.installer
+            - @ext.manager
             - phpbb-extension
             - EXTENSIONS_
 
diff --git a/phpBB/phpbb/composer/extension_manager.php b/phpBB/phpbb/composer/extension_manager.php
new file mode 100644
index 0000000000..ee3ac1aeaf
--- /dev/null
+++ b/phpBB/phpbb/composer/extension_manager.php
@@ -0,0 +1,63 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+namespace phpbb\composer;
+
+use phpbb\composer\exception\runtime_exception;
+use phpbb\extension\manager as ext_manager;
+
+/**
+ * Class to safely manage extensions through composer.
+ */
+class extension_manager extends manager
+{
+	/**
+	 * @var \phpbb\extension\manager
+	 */
+	protected $extension_manager;
+
+	/**
+	 * @param installer		$installer			Installer object
+	 * @param ext_manager	$extension_manager	phpBB extension manager
+	 * @param string		$package_type		Composer type of managed packages
+	 * @param string		$exception_prefix	Exception prefix to use
+	 */
+	public function __construct(installer $installer, ext_manager $extension_manager, $package_type, $exception_prefix)
+	{
+		$this->extension_manager = $extension_manager;
+
+		parent::__construct($installer, $package_type, $exception_prefix);
+	}
+
+	/**
+	 * {@inheritdoc}
+	 */
+	public function install(array $packages)
+	{
+		$packages = $this->normalize_version($packages);
+
+		$already_managed = array_intersect(array_keys($this->get_managed_packages()), array_keys($packages));
+		if (count($already_managed) !== 0)
+		{
+			throw new runtime_exception($this->exception_prefix, 'ALREADY_INSTALLED', [implode('|', $already_managed)]);
+		}
+
+		$installed_manually = array_intersect(array_keys($this->extension_manager->all_available()), array_keys($packages));
+		if (count($installed_manually) !== 0)
+		{
+			throw new runtime_exception($this->exception_prefix, 'ALREADY_INSTALLED_MANUALLY', [implode('|', array_keys($installed_manually))]);
+		}
+
+		$this->do_install($packages);
+	}
+}
diff --git a/phpBB/phpbb/composer/installer.php b/phpBB/phpbb/composer/installer.php
index 05c3ef2d68..7256e82d3a 100644
--- a/phpBB/phpbb/composer/installer.php
+++ b/phpBB/phpbb/composer/installer.php
@@ -86,7 +86,8 @@ class installer
 	{
 		$this->generate_ext_json_file($packages);
 
-		putenv('COMPOSER_VENDOR_DIR=' . $this->root_path . '/' . $this->packages_vendor_dir);
+		$original_vendor_dir = getenv('COMPOSER_VENDOR_DIR');
+		putenv('COMPOSER_VENDOR_DIR=' . $this->root_path . $this->packages_vendor_dir);
 
 		$io = new BufferIO('', OutputInterface::VERBOSITY_DEBUG);
 		$composer = Factory::create($io, $this->get_composer_ext_json_filename(), false);
@@ -123,6 +124,10 @@ class installer
 		{
 			throw new runtime_exception('Cannot install packages', [], $e);
 		}
+		finally
+		{
+			putenv('COMPOSER_VENDOR_DIR=' . $original_vendor_dir);
+		}
 	}
 
 	/**
@@ -134,10 +139,12 @@ class installer
 	 */
 	public function get_installed_packages($type)
 	{
+		$original_vendor_dir = getenv('COMPOSER_VENDOR_DIR');
+
 		try
 		{
 			$io = new NullIO();
-			putenv('COMPOSER_VENDOR_DIR=' . $this->root_path . '/' . $this->packages_vendor_dir);
+			putenv('COMPOSER_VENDOR_DIR=' . $this->root_path . $this->packages_vendor_dir);
 			$composer = Factory::create($io, $this->get_composer_ext_json_filename(), false);
 
 			$installed = [];
@@ -157,6 +164,10 @@ class installer
 		{
 			return [];
 		}
+		finally
+		{
+			putenv('COMPOSER_VENDOR_DIR=' . $original_vendor_dir);
+		}
 	}
 
 	/**
diff --git a/phpBB/phpbb/composer/manager.php b/phpBB/phpbb/composer/manager.php
index 836f39b509..4ade6e24a0 100644
--- a/phpBB/phpbb/composer/manager.php
+++ b/phpBB/phpbb/composer/manager.php
@@ -18,7 +18,7 @@ use phpbb\composer\exception\runtime_exception;
 /**
  * Class to manage packages through composer.
  */
-class manager
+class manager implements manager_interface
 {
 	/**
 	 * @var installer Composer packages installer
@@ -74,6 +74,16 @@ class manager
 			throw new runtime_exception($this->exception_prefix, 'ALREADY_INSTALLED', [implode('|', $already_managed)]);
 		}
 
+		$this->do_install($packages);
+	}
+
+	/**
+	 * Really install the packages.
+	 *
+	 * @param array $packages Packages to install.
+	 */
+	protected function do_install($packages)
+	{
 		$managed_packages = array_merge($this->get_managed_packages(), $packages);
 		ksort($managed_packages);
 
@@ -135,12 +145,12 @@ class manager
 	/**
 	 * Tells whether or not a package is managed by Composer.
 	 *
-	 * @param string $packages Package name
+	 * @param string $package Package name
 	 * @return bool
 	 */
-	public function is_managed($packages)
+	public function is_managed($package)
 	{
-		return array_key_exists($packages, $this->get_managed_packages());
+		return array_key_exists($package, $this->get_managed_packages());
 	}
 
 	/**
@@ -177,11 +187,15 @@ class manager
 	{
 		$normalized_packages = [];
 
-		foreach ($packages as $package)
+		foreach ($packages as $package => $version)
 		{
-			if (!is_array($package))
+			if (is_numeric($package))
 			{
-				$normalized_packages[$package] = '*';
+				$normalized_packages[$version] = '*';
+			}
+			else
+			{
+				$normalized_packages[$package] = $version;
 			}
 		}
 
diff --git a/phpBB/phpbb/composer/manager_interface.php b/phpBB/phpbb/composer/manager_interface.php
new file mode 100644
index 0000000000..d4c2b9eb51
--- /dev/null
+++ b/phpBB/phpbb/composer/manager_interface.php
@@ -0,0 +1,69 @@
+<?php
+/**
+ * Created by IntelliJ IDEA.
+ * User: tristan.darricau
+ * Date: 09/09/15
+ * Time: 23:12
+ */
+namespace phpbb\composer;
+use phpbb\composer\exception\runtime_exception;
+
+
+/**
+ * Class to manage packages through composer.
+ */
+interface manager_interface
+{
+	/**
+	 * Installs (if necessary) a set of packages
+	 *
+	 * @param array $packages Packages to install.
+	 *                        Each entry may be a name or an array associating a version constraint to a name
+	 *
+	 * @throws runtime_exception
+	 */
+	public function install(array $packages);
+
+	/**
+	 * Updates or installs a set of packages
+	 *
+	 * @param array $packages Packages to update.
+	 *                        Each entry may be a name or an array associating a version constraint to a name
+	 *
+	 * @throws runtime_exception
+	 */
+	public function update(array $packages);
+
+	/**
+	 * Removes a set of packages
+	 *
+	 * @param array $packages Packages to remove.
+	 *                        Each entry may be a name or an array associating a version constraint to a name
+	 *
+	 * @throws runtime_exception
+	 */
+	public function remove(array $packages);
+
+	/**
+	 * Tells whether or not a package is managed by Composer.
+	 *
+	 * @param string $packages Package name
+	 *
+	 * @return bool
+	 */
+	public function is_managed($packages);
+
+	/**
+	 * Returns the list of managed packages
+	 *
+	 * @return array The managed packages associated to their version.
+	 */
+	public function get_managed_packages();
+
+	/**
+	 * Returns the list of available packages
+	 *
+	 * @return array The name of the available packages, associated to their definition. Ordered by name.
+	 */
+	public function get_available_packages();
+}
diff --git a/phpBB/phpbb/console/command/extension/install.php b/phpBB/phpbb/console/command/extension/install.php
index f2358e7e4a..4d83fe2cef 100644
--- a/phpBB/phpbb/console/command/extension/install.php
+++ b/phpBB/phpbb/console/command/extension/install.php
@@ -14,6 +14,7 @@
 namespace phpbb\console\command\extension;
 
 use phpbb\composer\manager;
+use phpbb\composer\manager_interface;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
@@ -22,11 +23,11 @@ use Symfony\Component\Console\Style\SymfonyStyle;
 class install extends \phpbb\console\command\command
 {
 	/**
-	 * @var \phpbb\composer\manager Composer extensions manager
+	 * @var manager_interface Composer extensions manager
 	 */
 	protected $manager;
 
-	public function __construct(\phpbb\user $user, manager $manager)
+	public function __construct(\phpbb\user $user, manager_interface $manager)
 	{
 		$this->manager = $manager;
 
diff --git a/phpBB/phpbb/console/command/extension/list_available.php b/phpBB/phpbb/console/command/extension/list_available.php
index 0b20ce2d5e..107aca3410 100644
--- a/phpBB/phpbb/console/command/extension/list_available.php
+++ b/phpBB/phpbb/console/command/extension/list_available.php
@@ -17,6 +17,7 @@ use Composer\Package\CompletePackage;
 use Composer\Package\PackageInterface;
 use phpbb\composer\installer;
 use phpbb\composer\manager;
+use phpbb\composer\manager_interface;
 use Symfony\Component\Console\Formatter\OutputFormatter;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
@@ -26,11 +27,11 @@ use Symfony\Component\Console\Style\SymfonyStyle;
 class list_available extends \phpbb\console\command\command
 {
 	/**
-	 * @var \phpbb\composer\manager Composer extensions manager
+	 * @var manager_interface Composer extensions manager
 	 */
 	protected $manager;
 
-	public function __construct(\phpbb\user $user, manager $manager)
+	public function __construct(\phpbb\user $user, manager_interface $manager)
 	{
 		$this->manager = $manager;
 
diff --git a/phpBB/phpbb/console/command/extension/remove.php b/phpBB/phpbb/console/command/extension/remove.php
index a668322cdf..16a3ad263f 100644
--- a/phpBB/phpbb/console/command/extension/remove.php
+++ b/phpBB/phpbb/console/command/extension/remove.php
@@ -14,6 +14,7 @@
 namespace phpbb\console\command\extension;
 
 use phpbb\composer\manager;
+use phpbb\composer\manager_interface;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
@@ -22,11 +23,11 @@ use Symfony\Component\Console\Style\SymfonyStyle;
 class remove extends \phpbb\console\command\command
 {
 	/**
-	 * @var \phpbb\composer\manager Composer extensions manager
+	 * @var manager_interface Composer extensions manager
 	 */
 	protected $manager;
 
-	public function __construct(\phpbb\user $user, manager $manager)
+	public function __construct(\phpbb\user $user, manager_interface $manager)
 	{
 		$this->manager = $manager;
 
diff --git a/phpBB/phpbb/console/command/extension/update.php b/phpBB/phpbb/console/command/extension/update.php
index 01c9db0c28..e3f8935985 100644
--- a/phpBB/phpbb/console/command/extension/update.php
+++ b/phpBB/phpbb/console/command/extension/update.php
@@ -14,6 +14,7 @@
 namespace phpbb\console\command\extension;
 
 use phpbb\composer\manager;
+use phpbb\composer\manager_interface;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
@@ -22,11 +23,11 @@ use Symfony\Component\Console\Style\SymfonyStyle;
 class update extends \phpbb\console\command\command
 {
 	/**
-	 * @var \phpbb\composer\manager Composer extensions manager
+	 * @var manager_interface Composer extensions manager
 	 */
 	protected $manager;
 
-	public function __construct(\phpbb\user $user, manager $manager)
+	public function __construct(\phpbb\user $user, manager_interface $manager)
 	{
 		$this->manager = $manager;