test.php

<?php

class Foo
{
    public $name;

    public function __construct($name)
    {
        $this->name = $name;
    }
}

class Bar
{
    private $foo;

    public function __construct(Foo $foo = null, $default = 'default')
    {
        $this->foo = $foo ?: new Foo($default);
    }

    public function getFooName()
    {
        return $this->foo->name;
    }
}

$foo = new Foo('John');
$bar = new Bar($foo);
echo $bar->getFooName(); // John

$bar = new Bar();
echo $bar->getFooName(); // default

$bar = new Bar(null, 'something');
echo $bar->getFooName(); // something