How to check for a duplicate email address in PHP, considering Gmail (user.name+label@gmail.com)

Question

How can I check for duplicate email addresses in PHP, with the possibility of Gmail\'s automated labeler and punctuation in mind?

For example, I wan

Answer 1

This answer is an improvement on @powtac's answer. I needed this function to defeat multiple signups from same person using gmail.

if ( ! function_exists('normalize_email'))
{
    /**
     * to normalize emails to a base format, especially for gmail
     * @param $email
     * @return string
     */
    function normalize_email($email) {
        // ensure email is lowercase because of pending in_array check, and more...
        $email = strtolower($email);
        $parts    = explode('@', $email);

        // normalize gmail addresses
        if (in_array($parts[1], ['gmail.com', 'googlemail.com'])) {
            // check if there is a "+" and return the string before then remove "."
            $before_plus    = strstr($parts[0], '+', TRUE);
            $before_at      = str_replace('.', '', $before_plus ? $before_plus : $parts[0]);

            // ensure only @gmail.com addresses are used
            $email    = $before_at.'@gmail.com';
        }

        return $email;
    }
}