Starting with PHP as your choice for your backend web-applications is a great way to get you polishing with your skills with programming. The first thing you need to know in PHP is to need to know the following technologies.
CSS - for designing forms and HTML elements.
HTML - backbone for creating web pages.
The three mentioned above are just the starting point of getting your feet wet with PHP. Although, you don't have to master all three just to get PHP starting, but it is best that you have a through understanding when you start programming with PHP.
So What is PHP?
PHP is actually an acronym for "PHP Hypertext Preprocessor" and is widely used as an open source scripting language. PHP scripts are executed on the server-side. The real power why PHP is powerful enough to do all of your server-side functionality compared to other languages is the fact that PHP has over 1000 built-in functions that you can use with your program.
PHP can do amazing stuff, like read different types of databases, create,read, open, delete etc. on the server. PHP can also encrypt data, add, delete or modify databases and a lot more.