How to correctly reference charset = utf8

I have my coding in my php code in the PDO connection, it works correctly, but when you insert the data into MySQL, you can see it; (Ñ) Send them in lowercase CASTAñEDA example, in my form in most of the fields, I have what is $ strtoupper but I put all in lowercase and all converted to uppercase minus the tildes that I mention, I know that it is the case, some people say that they do not go with an accent but for example I put a character like this -> (Á, Ó) and similarly converts them into the base in (á, ó) example (Song) I do not know what my error is I'm investigating about $ mb_strtoupper but I really need an example in coding.

This is my code, in MySQL create the table with utf8_general_ci

<?php

$errores = NULL;
$enviado = FALSE;

if (isset($_POST['submit'])) {
$nombre =(empty(trim($_POST['nombre'])))  ? NULL :  strtoupper($_POST['nombre']);
$apellidopaterno = (empty(trim($_POST['apellidopaterno']))) ? NULL :  strtoupper($_POST['apellidopaterno']);
$apellidomaterno = (empty(trim($_POST['apellidomaterno']))) ? NULL :  strtoupper($_POST['apellidomaterno']);
$telefono = (empty(trim($_POST['telefono']))) ? NULL :   $_POST['telefono'];
$correo = (empty(trim($_POST['correo']))) ? NULL : $_POST['correo'];
$mensaje = (empty(trim($_POST['mensaje']))) ? NULL :  strtoupper($_POST['mensaje']);

 try{
                                  //codificación
    $conexion = new PDO ('mysql:host=localhost;dbname=pruebas;charset=utf8', 'root', '');

}catch (PDOException $e) {
    echo    "Error:" . $e->getMessage();
}

try{

    if (!$nombre){
        $errores .= 'Por favor ingresa tu nombre <br />';
    }

    if (!$apellidopaterno){
        $errores .= 'Por favor ingresa tu apellido paterno <br />';
    }

    if (!$apellidomaterno){
        $errores .= 'Por favor ingresa tu apellido materno <br />';
    }

    if (!$telefono){
        $errores .= 'Por favor ingresa tu teléfono <br />';
    }

    if (!$correo){
        $errores .= 'Por favor ingresa tu correo <br />';
    }

    if (!$mensaje){
        $errores .= 'Por favor ingresa un mensaje <br />';
    }

    if (!$errores){

        $statement = $conexion->prepare("INSERT INTO usuario(nombre,apellidopaterno,apellidomaterno,telefono,correo,mensaje)VALUES( :nombre, :apellidopaterno,:apellidomaterno, :telefono, :correo, :mensaje)");
        $statement->bindParam(':nombre', $nombre);
        $statement->bindParam(':apellidopaterno', $apellidopaterno);
        $statement->bindParam(':apellidomaterno', $apellidomaterno);
        $statement->bindParam(':telefono', $telefono);
        $statement->bindParam(':correo', $correo);
        $statement->bindParam(':mensaje', $mensaje);
        $conexion->beginTransaction();
        $statement->execute();
        $conexion->commit();
        $enviado=TRUE;
    }

}catch(Exception $e) {
    $conexion->rollback();
    echo "Error: 0" . $e->getMessage();
}


}

require 'index.view.php';



?>