PHPDoc of Array Key ?

Foo.php

class Foo 
{
    /**
     * @type array [
     *  @key string Bar ID (MD5)
     *  @value Bar $bar
     * ]
     * 
     * OR
     * 
     * @type array (
     *  @type string $key Bar ID (MD5) => @type Bar $value
     * )
     * 
     * ???
     */
    private $bars;
    
    public function addBar(Bar $bar)
    {
        $this->bars[$bar->getId()] = $bar;
    }
    
    public function findBarById($barId)
    {
        /**
         * Thanks to the fact I know $this->bars Key is Bar::id I don't have to iterate through the whole array
         * and check if ($barId === $bar->getId())
         */
        if (isset($this->bars[$barId])) {
            return $this->bars[$barId];
        }
        
        return false;
    }
}

What about:

<?php

class Foo
{
    /**
     * @var array of Bar indexed by its ID
     */
    private $bars;
}

Why not:

<?php

class Foo
{
    /**
     * @var Bar[string]
     */
    private $bars;
}