Simple interfaces to convert objects to scalar types and usage examples

interfaces.php

<?php
interface CastsToString {
    public function __toString(): string;
}

interface CastsToArray {
    public function __toArray(): array;
}

interface CastsToBool { // or CastsToBoolean
    public function __toBool(): bool;
}

interface CastsToInt { // or CastsToInteger
    public function __toInt(): int;
}

interface CastsToFloat {
    public function __toFloat(): float;
}

Money.php

<?php
// I left an example in the mailing list with a Money object
class Money implements CastsToString, CastsToFloat {
    const DOLLARS = '$';
    const POUND = '£';
    const YEN = '¥';
    
    private $currency;
    private $amount;
    
    public function __construct($amount, $currency){
        $this->amount = $amount;
        $this->currency = $currency;
    }
    
    public function __toString(){
        return $this->currency . $this->amount;
    }
    
    public function __toFloat(){
        return $this->amount;
    }
}

usage.php

<?php
// We could do stuff like:
$foo = new Foo(5);   // implements CastsToInt
$bar = pow($foo, 2); // internally calls $foo->__toInt()

$collection = new Collection();  // implements CastsToArray
array_walk($collection, 'trim');

if (new CollectionRule($collection)) { // implements CastsToBool
    // this one may be tricky, as now objects always evaluate to TRUE. 
    // But not BC break, as no code implements this interfaces yet
    echo 'great!';
}

// Usage of the Money class
$money = new Money(12.5, Money::YEN);

var_dump((string) $money); // string(5) "¥12.5"
var_dump((float)$money); // float(12.5)

Great Idea! But doesn't the ArrayAccess-interface cover the CastToArray-Interface-idea?

@heiglandreas: Not really. I just responded to francois in the mailing list with an example of Traversable, but ArrayAccess aplies all the same: if a method hints for array, the object won't pass the test. You'd have to iterate it somewhere in between messages, and build the resulting array outside of the object.

All of this gets stronger when you consider that our current userland ecosystem is very rich and we constantly use third party objects that we cannot (easily) modify.

I think this is very great idea 💯 but one thing which is hitting my eyes here is the fact that arrays are not scalars :-)