The PDO extension in PHP is a database access interface that provides an abstraction layer for interacting with databases from PHP applications.

The routing in PHP is the practice of defining how URLs will be associated with actions we perform in our web application. This is useful when using the MVC design pattern, for example: I enter the main page /, call the controller that will determine what to display, a landing page, an introduction, or perhaps a list of the latest topics.

The PHP function array_change_key_case changes the keys to uppercase or lowercase depending on whether we specify CASE_UPPER or CASE_LOWER.

The PHP function array_unshift inserts one or more elements at the beginning of the array.

The PHP function array_shift will remove the first element from the array and return its value. Note that changes to the array are made in the original array, so when we call the array again, it will no longer have its first element.