Type hint stupidity

hints.php

<?php

// PROBLEM: I want to use type-hints to make sure the parameters of
// a function are all the right type.

function printColor(Color $theColor) {
    echo "The color is " . $theColor->getValue() . PHP_EOL;
}

interface ValueType {
    public static function getValue();
}

interface Color extends ValueType {}

class Red implements Color {
    public static function getValue() {
        return 'red';
    }
}

class Blue implements Color {
    public static function getValue() {
        return 'blue';
    }
}

class Green implements Color {
    public static function getValue() {
        return 'green';
    }
}

// printColor('blue'); // FAILS
printColor(new Blue()); // WORKS!