Why can't I overload constructors in PHP?

Question

You can't overload ANY method in PHP. If you want to be able to instantiate a PHP object while passing several different combinations of parameters, use the factory pattern with a private constructor.

For example:

public MyClass {
    private function __construct() {
    ...
    }

    public static function makeNewWithParameterA($paramA) {
        $obj = new MyClass(); 
        // other initialization
        return $obj;
    }

    public static function makeNewWithParametersBandC($paramB, $paramC) {
        $obj = new MyClass(); 
        // other initialization
        return $obj;
    }
}

$myObject = MyClass::makeNewWithParameterA("foo");
$anotherObject = MyClass::makeNewWithParametersBandC("bar", 3);

You can use variable arguments to produce the same effect. Without strong typing, it doesn't make much sense to add, given default arguments and all of the other "work arounds."

For completeness, I'll suggest Fluent Interfaces. The idea is that by adding return $this; to the end of your methods you can chain calls together. So instead of

$car1 = new Car('blue', 'RWD');
$car2 = new Car('Ford', '300hp');

(which simply wouldn't work), you can do:

$car = (new Car)
       ->setColor('blue')
       ->setMake('Ford')
       ->setDrive('FWD');

That way you can pick exactly which properties you want to set. In a lot of ways it's similar to passing in an array of options to your initial call:

$car = new Car(['make' => 'Ford', 'seats' => 5]);

True overloading is indeed unsupported in PHP. As @Pestilence mentioned, you can use variable arguments. Some people just use an Associative Array of various options to overcome this.

PHP Manual: Function Arguments, Default Values

I have overcome this simply by using default values for function parameters. In __constuct, list the required parameters first. List the optional parameters after that in the general form $param = null.

class User
{
    private $db;
    private $userInput;

    public function __construct(Database $db, array $userInput = null)
    {
        $this->db = $db;
        $this->userInput = $userInput;
    }
}

This can be instantiated as:

$user = new User($db)

or

$user = new User($db, $inputArray);

This is not a perfect solution, but I have made this work by separating parameters into absolutely mandatory parameters no matter when the object is constructed, and, as a group, optional parameters listed in order of importance.

It works.