如何调用存储在不同文件夹和文件中的函数？

I am completely new on PHP, I crated two functions stored on a different folder and file. I would like to know how to call these functions on any file. Here my files.

\functions\funciones.php file

    <?php 
    //Comprueba si existe una sesión activa, si no redirecciona al index.php
    function ComprobarSesion () {
        if (isset($_SESSION['usuario'])){
        header('Location: index.php');
        }
    }
    // Funcion conexión a la DB. Validar si tambien se peude meter en una funcion. 
    function Conexion_DB(){
        try {
            $conexion = new PDO('mysql:host=127.0.0.1;dbname=login','root','');
        } catch (PDOException $e) {
            echo "Error: ".$e->getMessage();
        }
    }
    ?>

\index.php file

    <?php session_start();
    if (isset($_SESSION['USUARIO'])) {
        header('Location: contenido.php');
    }   else {
        header('Location: registro.php');
    }
    ?>

As you can see in the "index.php" file, I have the code that I want to remove and put the function... but I don't know how to replace it.

You could use require_once.

Put this at the top of your index-file

<?php 
require_once('/absolute/path/to/your/file.php');

Replace the path and desired file.

Then you can call the function in your index.php like this

ComprobarSesion();

Hope this helps :)