Lumen L5 compatibility helpers. That file should be added on root path of your project... and added to your composer.json

compat_l5.php

<?php

if(!function_exists('config_path'))
{
        /**
        * Return the path to config files
        * @param null $path
        * @return string
        */
        function config_path($path=null)
        {
                return app()->getConfigurationPath(rtrim($path, ".php"));
        }
}

if(!function_exists('public_path'))
{

        /**
        * Return the path to public dir
        * @param null $path
        * @return string
        */
        function public_path($path=null)
        {
                return rtrim(app()->basePath('public/'.$path), '/');
        }
}

if(!function_exists('storage_path'))
{

        /**
        * Return the path to storage dir
        * @param null $path
        * @return string
        */
        function storage_path($path=null)
        {
                return app()->storagePath($path);
        }
}

if(!function_exists('database_path'))
{

        /**
        * Return the path to database dir
        * @param null $path
        * @return string
        */
        function database_path($path=null)
        {
                return app()->databasePath($path);
        }
}

if(!function_exists('resource_path'))
{

        /**
        * Return the path to resource dir
        * @param null $path
        * @return string
        */
        function resource_path($path=null)
        {
                return app()->resourcePath($path);
        }
}

if(!function_exists('lang_path'))
{

        /**
        * Return the path to lang dir
        * @param null $str
        * @return string
        */
        function lang_path($path=null)
        {
                return app()->getLanguagePath($path);
        }
}

if ( ! function_exists('asset'))
{
	/**
	 * Generate an asset path for the application.
	 *
	 * @param  string  $path
	 * @param  bool    $secure
	 * @return string
	 */
	function asset($path, $secure = null)
	{
		return app('url')->asset($path, $secure);
	}
}

if ( ! function_exists('elixir'))
{
    /**
     * Get the path to a versioned Elixir file.
     *
     * @param  string  $file
     * @return string
     */
    function elixir($file)
    {
        static $manifest = null;
        if (is_null($manifest))
        {
            $manifest = json_decode(file_get_contents(public_path().'/build/rev-manifest.json'), true);
        }
        if (isset($manifest[$file]))
        {
            return '/build/'.$manifest[$file];
        }
        throw new InvalidArgumentException("File {$file} not defined in asset manifest.");
    }
}

if ( ! function_exists('auth'))
{
	/**
	 * Get the available auth instance.
	 *
	 * @return \Illuminate\Contracts\Auth\Guard
	 */
	function auth()
	{
		return app('Illuminate\Contracts\Auth\Guard');
	}
}

if ( ! function_exists('bcrypt'))
{
	/**
	 * Hash the given value.
	 *
	 * @param  string  $value
	 * @param  array   $options
	 * @return string
	 */
	function bcrypt($value, $options = array())
	{
		return app('hash')->make($value, $options);
	}
}

if ( ! function_exists('redirect'))
{
	/**
	 * Get an instance of the redirector.
	 *
	 * @param  string|null  $to
	 * @param  int     $status
	 * @param  array   $headers
	 * @param  bool    $secure
	 * @return \Illuminate\Routing\Redirector|\Illuminate\Http\RedirectResponse
	 */
	function redirect($to = null, $status = 302, $headers = array(), $secure = null)
	{
		if (is_null($to)) return app('redirect');
		return app('redirect')->to($to, $status, $headers, $secure);
	}
}

if ( ! function_exists('response'))
{
	/**
	 * Return a new response from the application.
	 *
	 * @param  string  $content
	 * @param  int     $status
	 * @param  array   $headers
	 * @return \Symfony\Component\HttpFoundation\Response|\Illuminate\Contracts\Routing\ResponseFactory
	 */
	function response($content = '', $status = 200, array $headers = array())
	{
		$factory = app('Illuminate\Contracts\Routing\ResponseFactory');
		if (func_num_args() === 0)
		{
			return $factory;
		}
		return $factory->make($content, $status, $headers);
	}
}

if ( ! function_exists('secure_asset'))
{
	/**
	 * Generate an asset path for the application.
	 *
	 * @param  string  $path
	 * @return string
	 */
	function secure_asset($path)
	{
		return asset($path, true);
	}
}

if ( ! function_exists('secure_url'))
{
	/**
	 * Generate a HTTPS url for the application.
	 *
	 * @param  string  $path
	 * @param  mixed   $parameters
	 * @return string
	 */
	function secure_url($path, $parameters = array())
	{
		return url($path, $parameters, true);
	}
}


if ( ! function_exists('session'))
{
	/**
	 * Get / set the specified session value.
	 *
	 * If an array is passed as the key, we will assume you want to set an array of values.
	 *
	 * @param  array|string  $key
	 * @param  mixed  $default
	 * @return mixed
	 */
	function session($key = null, $default = null)
	{
		if (is_null($key)) return app('session');
		if (is_array($key)) return app('session')->put($key);
		return app('session')->get($key, $default);
	}
}


if ( ! function_exists('cookie'))
{
	/**
	 * Create a new cookie instance.
	 *
	 * @param  string  $name
	 * @param  string  $value
	 * @param  int     $minutes
	 * @param  string  $path
	 * @param  string  $domain
	 * @param  bool    $secure
	 * @param  bool    $httpOnly
	 * @return \Symfony\Component\HttpFoundation\Cookie
	 */
	function cookie($name = null, $value = null, $minutes = 0, $path = null, $domain = null, $secure = false, $httpOnly = true)
	{
		$cookie = app('Illuminate\Contracts\Cookie\Factory');
		if (is_null($name))
		{
			return $cookie;
		}
		return $cookie->make($name, $value, $minutes, $path, $domain, $secure, $httpOnly);
	}
}

Can you give directions on how to include this file in composer.json and where to locate the helper file concerning best practices ?
That will be helpful for newbies in lumen.

Thanks for your contribution.

This article shows how to do it using composer:
https://laravel-news.com/creating-helpers