<?php

class MyString{
    function __construct($string){
        $this->storage = $string;
    }
    function substr($from,$len){
        return new static(substr($this->storage, $from, $len));
    }
    static function __fromScalar($value){
        return new static($value);
    }
}

function test(MyString $value){
    echo $value->substr(0,5);
}

test("hello world"); // implicitly constructs the object via MyString::__fromScalar() and prints "hello"

// ---------------------------------------------------------------------

class Color
{
    function __construct($red, $green, $blue, Integer $alpha = 1){
        // ...
    }
    static function newFromHex(String $string){
        // ...
    }
    static function newFromHSV(...){
        // ...
    }
    static function newFromHSL(...){
        // ...
    }
    static function __fromScalar($value){
        if(preg_match("@^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$@i", $value))
            return static::newFromHex($value);
        elseif(preg_match(...))
            return static::newFromHSL(.....);
        elseif(preg_match(...))
            return static::newFromHSV(.....);
        // ...
        else
            throw new Exception("couldn't parse the color string");
    }
    function setAlpha(Integer $alpha){
        $this->alpha = $alpha;
    }
    function __toString(){
        // ...
    }
}

function test(Color $color){
    return $color->setAlpha(0.9);
}

test("#ff0000"); // prints rgba(255,0,0,0.9)
test("rgb(255,0,0)"); // prints rgba(255,0,0,0.9)


// ---------------------------------------------------------------------

// also:

// method return type:
function ReturnTypeClass test(String $test){
    return "hello"; // correct, equals to write "return ReturnTypeClass::__fromScalar("hello");"
}

class ReturnTypeClass{
    static function __fromScalar($value){
        return new self($value);
    }
}