在CakePHP公共类变量中连接常量和字符串

How do I concatenate global constants and strings in model $actsAs arrays in CakePHP 2.6?

I've got a file upload plugin that needs directory paths fed to it via the $actsAs configuration, but while it lets me just use a global constant OR a string as an array key value, it doesn't let me concatenate two together. This is a problem because theoretically my app could be living in various environments where the path to the app is different, and I'd rather not have this hard-coded and instead use a path relative to the global constant ROOT.

It looks to me like I'm running up against a limitation of PHP. What good solutions are there to this? Cake-specific / Cake-friendly solutions are a plus.

What works:

// Import.php
public $actsAs = array(
    'Uploader.Attachment' => array(
        'filename' => array(
            // You can use a global constant by itself:
            'tempDir' => TMP,
            'uploadDir' => 
            // Using a full string works, too.
            '/var/www/vhosts/hostname/app/webroot/file/imports',
            //
            'finalPath' => '/file/imports/',
            'nameCallback' => 'nameCallback',
            'overwrite' => false,
        ),
    ),
);

What I'd like to work, but doesn't work. Instead of concatenating, I get an "unexpected '.'" error:

public $actsAs = array(
    'Uploader.Attachment' => array(
        'filename' => array(
            'tempDir' => TMP,
            // Concatenation of the two results in an "unexpected ." error:
            // ROOT being '/var/www/vhosts/hostname/'
            'uploadDir' => ROOT . '/app/webroot/file/imports',
            //
            'finalPath' => '/file/imports/',
            'nameCallback' => 'nameCallback',
            'overwrite' => false,
        ),
    ),
);

You can do that in the constructor:

public function __construct($id = false, $table = null, $ds = null) {
   $this->actAs = array(
    'Uploader.Attachment' => array(
        'filename' => array(
            'tempDir' => TMP,
            'uploadDir' => ROOT . '/app/webroot/file/imports',
            'finalPath' => '/file/imports/',
            'nameCallback' => 'nameCallback',
            'overwrite' => false,
        ),
    ),
   );
  parent::__construct($id, $table, $ds);
}