Wordpress get plugin directory

Is there any function that would return the full path of my plugin in WordPress?

Example is

path/wp-contents/plugins/myplugin

I have tried plugin_dir_path(__FILE__) but returns the current dir.

I would suggest to use a WordPress internal constant to solve this case:

$my_plugin = WP_PLUGIN_DIR . '/my-plugin';

if ( is_dir( $my_plugin ) ) {
    // plugin directory found!
}

Alternative

The other valid alternative is to compute the path from the URL which is more complex/confusing. I would not use this code:

$plugins_url = plugins_url();
$base_url = get_option( 'siteurl' );
$plugins_dir = str_replace( $base_url, ABSPATH, $plugins_url );
// Now $plugins_dir is same as the WP_PLUGIN_DIR constant.

$my_plugin = $plugins_dir . '/my-plugin';

My opinion in this case is: Use the constant WP_PLUGIN_DIR

Yeah as per description of plugin_dir_path it will give you current plugin file path. But as per what you asking here you can do something like below unfortunately no direct way,

$plugin_dir = ABSPATH . 'wp-content/plugins/plugin-folder/';

Edit: 18-09-2021

The best practice is to use with latest version is WP_PLUGIN_DIR as follow:

$plugin_dir = WP_PLUGIN_DIR . '/plugin-folder';

Code For Plugin Root Path

$dir = plugin_dir_path( __FILE__ );

// Example: /home/user/var/www/wordpress/wp-content/plugins/my-plugin/

Code for plugin path

echo  WP_PLUGIN_DIR.'/plugin-name';

Kinda late to this party, but just in case some else stumbles upon this.

plugin_dir_path(__FILE__) will always return the current path (where the file calling it is located).
If you want the root, use the code below:

plugin_dir_path( dirname( __FILE__ ) )

You can then define a constant:

define( 'YOUR_PLUGIN_DIR', plugin_dir_path( dirname( __FILE__ ) ) );
require_once YOUR_PLUGIN_DIR . 'includes/admin-page.php'
require_once YOUR_PLUGIN_DIR . 'admin/classes.php'