<?php
class DomainValidator {
    const DOMAIN_TAG = '@domain';

    private $object;

    public function __construct($object) {
        $this->object = $object;
    }

    public function __call($function, $arguments) {
        if (!$this->verify_domain($function, $arguments)) {
            throw new \DomainException('Bad domain!');
        }

        return call_user_func_array(
            array($this->object, $function),
            $arguments
        );
    }

    public function __get($name) {
        return $this->object->name;
    }

    public function __set($name, $value) {
        $this->object->name = $value;
    }

    private function verify_domain($function, $arguments) {
        // Get reference to method
        $method = new \ReflectionMethod($this->object, $function);
        $domains = $this->get_domains($method->getDocComment());
        $arguments = $this->parse_arguments(
            $method->getParameters(),
            $arguments
        );
        foreach ($domains as $domain) {
            if (!call_user_func(
                $domain['name'],
                $arguments[$domain['parameter']]
            )) {
                return false;
            }
        }
        return true;
    }

    private function get_domains($doc_block) {
        $lines = explode("\n", $doc_block);
        $domains = array();
        $domain_tag = DomainValidator::DOMAIN_TAG . ' ';
        foreach ($lines as $line) {
            $has_domain = stristr($line, $domain_tag) !== false;
            if ($has_domain) {
                $domain_info = explode($domain_tag, $line);
                $domain_info = explode(' ', $domain_info[1]);
                $domains[] = array(
                    'name'      => $domain_info[0],
                    'parameter' => $domain_info[1],
                );
            }
        }
        return $domains;
    }

    private function parse_arguments($parameters, $values) {
        $ret = array();
        for ($i = 0, $size = sizeof($values); $i < $size; $i++) {
            $ret[$parameters[$i]->name] = $values[$i];
        }
        return $ret;
    }
}

class FooDomain {
    public static function is_not_empty($input) {
        return !empty($input);
    }
}

class Foo {
    /**
     * @domain FooDomain::is_not_empty my_string
     */
    public function print_string($my_string) {
        echo $my_string . PHP_EOL;
    }
}

$foo = new DomainValidator(new Foo());
$foo->print_string('Hello, world!');
try {
    $foo->print_string(''); // throws a DomainException
} catch (\DomainException $e) {
    echo 'Could not print an empty string...' . PHP_EOL;
}