I have been doing web development with PHP. How do I start web development with PHP?
The question is a bit confusing but I think you are asking for on how to start in applying PHP into some kind of a project.
Not only for PHP but for any Web development language/framework start with a CRUD Application.
CRUD: Create, Read, Update , Delete.
I've followed this for PHP,Ruby on Rails , Firebase . And it is always fun to achieve simple CRUD app in a Tech which your are exploring.
Like a blog system. Where you can create/submit your blog article(create), Read the list of blogs you have submitted(Read), Edit and update the already published blog(update) and delete particular already published blog article(delete).
In mysql + php , INSERT INTO statement does the work of Creation, SELECT statement does the work to read, UPDATE statement does the work to edit and update and DELETE does the work to delete(obvio ;-p).
For all this you need to have basic knowledge of PHP Forms and PHP&MySQL Connection. (Both the things are very easy and i hope you are considering MySQL for DB).