class design improvement - user validation
First of all, as mentioned by @user7212, use php filter_var() for email validation. Please see below for example:

/**
 * Validate email
 *
 * @param string $email
 *
 * @return bool
 */
public static function email($email)
{
    return filter_var($email, FILTER_VALIDATE_EMAIL);
}


Second, why not naming your function params? If the email validation function would be called $email, if would be much easier to understand then $a.

in_array() function, according to it's documentation, returns bool. Why converting it to int? The result of the check should be bool.

In the pass() function, are you checking a string or a hash? Which brings us to the following item - write comments for your code! PHPDoc is a good example on how to do that.

And the last, but not the least - the name of the class. Check is not the most appropriate name. I would consider Validation or Valid.

Hope this helps.