It is written in php. If you already know php, it will be somewhat easier for you to grasp. You'll have to learn the wordpress architecture and its library.

You can find the wordpress socumentation at https://developer.wordpress.org/

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WordPress is no doubt written in PHP language but its a combination of different server side and markup languages like HTML, CSS, jQuery, Javascript, PHP as well as OOP terminology.

Language like PHP is not very difficult to grasp but I would recommend you to focus on basics, if you do good knowledge of PHP basics and follow best practice then I assure you its not difficult to be a master mind developer of WordPress.

But if you need some help or want to develop custom wordpress or child theme or plugin, you can hire developers like me.

This doesn't make any sense.

Anyone that knows PHP and has internet access knows Wordpress includes PHP. All you had to do is look.

...or post here and never return.

It is based on PHP, but the pre defined function and their way of using make it different than others. If you are a php coder then you can easily understand and implement the code.

WordPress is built on PHP, MySQL. WordPress uses client side markup like HTML, CSS, Jquery, Javascript. PHP is very basic language and easy to learn.

You can learn from W3Schools - https://www.w3schools.com/php/

Wordpress is written in PHP, and it is a combination of serverside programming languages(PHP) and markup languages like HTML, CSS, Javascript,Jquery

In PHP and MySQL database. For developing in WP, you want to start with WP Codex.

Wordpress is written in PHP language. We should have the knowledge of server side scripting languages like JavaScript, JQuery, Object oriented Programming.
PHP is very simple to learn if we have basic knowledge of HTML and JavaScript.