Are you certain that you want it to be online?
The problem that I foresee with that is that it might be ok for single pages, but PHP apps tend to chain from page to page. E.g. show login page, on click submit button, go to validation page, from which go to either error page & page to login, or to main app page (from where, possibly go to other pages).
I would recommend you to learn to develop on your local machine. If Linux, use Apache, if Windows, use Xampp, which provides an Apache server.
That way, you can launch your code in the browser as
http://localhost/my_app.php, where Apache provides
For IDEs, you can use the free Eclipse PHP, or NetBeans, or CodeLobster. I recommend them in that order, but YKmMV.
There are other free IDFEs, so please search or listen to other recommendations.
Since you can budget $10/month, I think that you simply cannot beat JetBrains PHPstorm. Their pricing is
£149.00/user 1st year
£89.00/3rd year onwards
Teach yourself to use the debugger – it is your best friend. Set a breakpoint at a line in your code & run the code. It will stop on the breakpoint and you can examine the call stack and the values of your variables (even change them, for testing).
I recommend that you learn MySql (or MAraiaDB) of your database and use PDO to access it.
I know that you asked for an online solution, but after 20 years of developing PHP on my local machine, I strongly recommend this way. Good luck