Roundcube is a free software webmail.
I need to implement an HTML class to connect Roundcube with MySQL database.
I have a file that is in skin / larry / template / login.html, the idea was to change the login page. Write the entire page. Now I am not understanding how the roundcube already connects to the database. Which functions should I use?
Dude, I think you're a bit confused .. There is no "HTML class", HTML is a markup language only, it is used to define and display the site in the browser, to make a connection to a database you need of a programming language, in this case, PHP (the language in which the RoundCube is made).
In the case you mentioned they seem to use a template language to render the pages of the system, analyzed the page you mentioned, I saw that the form is created in that section:
<roundcube:form name="form" method="post">
<roundcube:object name="loginform" form="form" size="40" submit=true />
</form>
I do not know what template language is being used, but taking a quick look at it implies that you are calling this method:
protected function login_form($attrib)
On the file: program / include / rc_mail_output_html.php
In short: You can use the HTML above to display the login form in a custom template, now to customize the form itself, or change the function that is being executed in the form submit is already much more complicated.
Maybe you should look for documentation on the RoundCube website, or contact them to learn how to better customize your own template.