How to get root directory path in a phtml magento 2?

Class \Magento\Framework\Filesystem\DirectoryList is used to get path like root, media, var etc

This will get root directory of your project like this

/var/www/html/myproject

By ObjectManager

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();

$directory = $objectManager->get('\Magento\Framework\Filesystem\DirectoryList');

echo $rootPath  =  $directory->getRoot();

By Dependency Injection

protected $_dir;
...

public function __construct(
    ...
    \Magento\Framework\Filesystem\DirectoryList $dir,
    ...        
) {
    ...
    $this->_dir = $dir;
    ...
}

Get other directory paths like

$this->_dir->getRoot(); // Output: /var/www/html/myproject

$this->_dir->getPath('media'); // Output: /var/www/html/myproject/pub/media

$this->_dir->getPath('pub'); // Output: /var/www/html/myproject/pub

$this->_dir->getPath('static'); // Output: /var/www/html/myproject/pub/static

$this->_dir->getPath('var'); // Output: /var/www/html/myproject/var

$this->_dir->getPath('app'); // Output: /var/www/html/myproject/app

$this->_dir->getPath('etc'); // Output: /var/www/html/myproject/app/etc

$this->_dir->getPath('lib_internal'); // Output: /var/www/html/myproject/lib/internal

$this->_dir->getPath('lib_web'); // Output: /var/www/html/myproject/lib/web

$this->_dir->getPath('tmp'); // Output: /var/www/html/myproject/var/tmp

$this->_dir->getPath('cache'); // Output: /var/www/html/myproject/var/cache

$this->_dir->getPath('log'); // Output: /var/www/html/myproject/var/log

$this->_dir->getPath('session'); // Output: /var/www/html/myproject/var/session

$this->_dir->getPath('setup'); // Output: /var/www/html/myproject/setup/src

$this->_dir->getPath('di'); // Output: /var/www/html/myproject/var/di

$this->_dir->getPath('upload'); // Output: /var/www/html/myproject/pub/media/upload

$this->_dir->getPath('generation'); // Output: /var/www/html/myproject/var/generation

$this->_dir->getPath('view_preprocessed'); // Output: /var/www/html/myproject/var/view_preprocessed

$this->_dir->getPath('composer_home'); // Output: /var/www/html/myproject/var/composer_home

$this->_dir->getPath('html'); // Output: /var/www/html/myproject/var/view_preprocessed/html

NOTE: You should never use \Magento\Framework\App\ObjectManager::getInstance() It defeats the purpose of dependency injection.

You can use magento 2 default variable to get absolute path of Magento directory

echo BP;

You have to use ROOT for get root directory

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();

$storeManager = $objectManager->get('\Magento\Store\Model\StoreManagerInterface');
$fileSystem = $objectManager->get('Magento\Framework\Filesystem');
$mediaPath  =   $fileSystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::ROOT)->getAbsolutePat‌​h()