lazy_string.php

class LazyString
{
    /**
     * @var callable
     */
    private $callee;

    /**
     * @param callable $callee
     */
    public function __construct(callable $callee)
    {
        $this->callee = $callee;
    }

    public function __toString()
    {
        return call_user_func($this->callee);
    }
}

$lazyString = new LazyString(function () {
    return some_heavy_string_function();
});

echo $lazyString;