WordPress - Functions 

At the core of WordPress theming is a file called functions.php - that's where we define our functions and use WordPress hooks.
In this video - we cover the basics by defining a function inside the functions.php file and calling that function in another file within our theme.
Excerpt from the WordPress docs:
The functions.php file is where you add unique features to your WordPress theme. It can be used to hook into the core functions of WordPress to make your theme more modular, extensible, and functional.
What is functions.php? #What is functions.php?
The functions.php file behaves like a WordPress plugin, adding features and functionality to a WordPress site. You can use it to call WordPress functions and to define your own functions.
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Interesting in learning more? Check out the docs below.
developer.word...
Here's documentation not specific to WordPress - but specific to PHP and user-defined functions.
www.php.net/ma...
www.w3schools....