Just installed Devtools using composer on my Windows OS.

My PHP version: php -v

PHP 5.6.3 (cli) (built: Nov 12 2014 17:18:08)
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2014 Zend Technologies

Then tried to execute phalcon webtools enable : 

Phalcon DevTools (3.2.3)

Fatal error: Default value for parameters with a class type hint can only be NULL in D:\xampp\...\vendor\phalcon\devtools\scripts\Phalcon\Commands\DotPhalconMissingException.php on line 37

Inspecting the file:

[36]    ...
[37]    public function __construct (string $message = self::DEFAULT_MESSAGE , $code = 0)
[38]    ...

Then removed the class type hint ...

[36]    ...
[37]    public function __construct ($message = self::DEFAULT_MESSAGE , $code = 0)
[38]    ...

... and it worked fine.

Phalcon DevTools (3.2.3)

  Info: This command must be run inside a Phalcon project with a .phalcon directory. One was not found at D:\xampp\...

  Info: Shall I create the .phalcon directory now? (y/n)

y

Retrying command...

  Success: Webtools successfully enabled!