PHP - member encapsulation returns strange reference

class ContentHolder
{
    private $content;
    public function __construct()
    {
        $this->content = "";
    }

    public function getContent()
    {
        return $this->content;
    }
}
$c = new ContentHolder();
$foo = array();
$foo['c'] = $c->getContent();

public function &getContent()
{
    return $this->content;
}

$foo['c'] = &$c->getContent();

public function __construct() {
    $this->content = "test";
}

$c = new ContentHolder();
$foo = array();
$foo['c'] = $c->getContent();

print $foo['c'];          // prints "test"
print $c->getContent();   // prints "test"

var_dump($foo);

array(1) {
  ["c"]=>
  string(0) ""
}

echo "foo[c] = '" . $foo['c'] . "'n";

foo[c] = ''