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)

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