You have an option to write your own HTML and CSS code in here.
When you are writing HTML codes, make sure, that they are valid, because if they aren't, your slide won't be saved down correctly, which will mess up your slider, and you will need to remake it!
You can't put PHP code into our backend. You should either just link to a PHP file, where you could do what you want, and redirect back, or if you know more advanced code developing, you could use AJAX too.
Please note that we don't support third party codes in any case.