How to call the constructor with call_user_func_array in PHP

I think for now the only way to do what you want is:

class MyHelloWorld extends Base {

    use SayWorld {
        SayWorld::__construct as private __swConstruct;
    }

    public function __construct($a, $b, $c = 0)
    {
        $this->__swConstruct($a, $b, $c);
    }
}

Edit 2:

My advice, based on over a year of dealing with traits in PHP, is: avoid writing constructors in traits at all, or if you must - at least make them parameterless. Having them in traits goes against the idea of constructors in general, which is: constructors should be specific to a class to which they belong. Other, evolved high-level languages don't even support implicit constructor inheritance. This is because constructors have far more stronger relation to the class then other methods. In fact they have so strong relation, that even the LSP does not apply to them. The traits in Scala language (a very mature and SOLID-friendly successor of Java), can't have a constructor with parameters.

Edit 1:

There was a bug in PHP 5.4.11, which actually allowed to alias a superclass method. But this was considered a no-no by the PHP developers, so we are still stuck with that cumbersome solution which I presented above. But that bug raised a discussion about what can be done with this, and I'm hoping it will be targeted in future releases.

Meanwhile I came across the same problem over and over again. My irritation raised exponentially with the number of parameters and lines of docblock which had to be repeated a lot of times in order to use the trait. So I came up with the following pattern in order to stick to the DRY rule as much as I could:

Instead of repeating entire set of parameters like this:

trait SayWorld {

    /**
     * This is a valid docblock.
     *
     * @param int $a Doc comment.
     * @param int $b Doc comment.
     */
    public function __construct($a, $b) {
        echo (int)$c * ($a+$b);
    }
}

class MyHelloWorld extends Base {

    use SayWorld {
        SayWorld::__construct as private __swConstruct;
    }

    /**
     * Repeated and unnecessary docblock.
     *
     * @param int $a Doc comment.
     * @param int $b Doc comment.
     * @param int $c Doc comment.
     */
    public function __construct($a, $b, $c = 0)
    {
        $this->__swConstruct($a, $b);
    }
}

I write a class much like a tuple (concept familiar to C# and Python users), and use it instead of an endless list of parameters:

class SayWorldConstructTuple
{
    public $a;

    public $b;

    public function __construct($a, $b)
    {
        $this->a = $a;
        $this->b = $b;
    }
}

class MyHelloWorld extends Base {

    use SayWorld {
        SayWorld::__construct as private __swConstruct;
    }

    /**
     * New and valid docblock.
     *
     * @param SayWorldConstructTuple $Tuple
     * @param int $c Additional parameter.
     */
    public function __construct(SayWorldConstructTuple $Tuple, $c = 0)
    {
        $this->__swConstruct($Tuple->a, $Tuple->b);
        $this->c = $c;
    }
}

Note: this pattern is of course more useful with a larger amount of tuple's constructor parameters, and more classes using the tuple.

It can be automated further with the use of PHP's dynamic nature.

Read all of Constructors and Destructors.

Constructors can take parameters like any other function or method in PHP:

class MyClass {

  public $param;

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

$myClass = new MyClass('foobar');
echo $myClass->param; // foobar

Your example of how you use constructors now won't even compile as you can't reassign $this.

Also, you don't need the curly brackets every time you access or set a property. $object->property works just fine. You only need to use curly-brackets under special circumstances like if you need to evaluate a method $object->{$foo->bar()} = 'test';