Easy arguments validation in php

Question

Every time you write some php functions or class methods, it's good practice to check input arguments and throw an exceptions, or trigger errors or warnings etc...

For example

<?php

function send_email($email, $subject, $body)
{
    if (empty($email)) {
        throw new InvalidArgumentException('Email should not be empty');
    }
    if (!is_string($email) || filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
        throw new InvalidArgumentException('Email format is invalid');
    }
    if (empty($subject)) {
        throw new InvalidArgumentException('Subject should not be empty');
    }
    if (!is_string($subject)) {
        throw new InvalidArgumentException('Subject must be a string');
    }
    if (empty($body)) {
        throw new InvalidArgumentException('Body should not be empty');
    }
    if (!is_string($body)) {
        throw new InvalidArgumentException('Body must be a string');
    }

    return mail($email, $subject, $body);
}

As you can see most of this example contains validation code, whereas only one helpful line which do the job. You actually need to write a lot of code if you want to reliably protect your functions. And this is tediously.

My question is - does anybody know some good way to validate code easily? Is there any libraries that validate depends on PHP-DOC? For example:

<?php

/**
 * @param email $email
 * @param string $subject
 * @param string $body
 */
function send_email($email, $subject, $body)
{
    return mail($email, $subject, $body);
}

Thank you.

Answer 1

The easiest way in pure PHP is to simplify the check by using assertions:

class Assert {

    public static function isString($var) {
        if (!is_string($var)) {
            throw new InvalidArgumentException('Argument is not a string');
        }
    }

}

function foo($string) {
    Assert::isString($string);
    ...
}

You can spruce this up with introspection and/or debug backtraces to include more information in the thrown exception.