HTML is definitely a handy asset when you start learning PHP, but it's not 'necessary' as such, you can write working PHP fine with no prior experience in HTML. However, as generally your PHP page would have some form of output that the user sees, having HTML to handle the way that output is presented is great. Javascript is even less 'required'. You only need to know jScript for the client side coding, so unless your page will require some event driven programming, it's not at all essential.
My personal recommendation is that PHP and HTML are like two peas in a pod, learn them both at the same time and the world will go around smoothly.