create uncrud localstorage and mysql

Good morning

I need your help, I'm making an application type of saved notes, that allows me to save the notes in the localstorage as in a mysql so that if the application is deleted you can keep the data on a server and thus never lose the saved notes

what happens is that I could create the part of the localstorage, but it is complicated when I pass the data to the server, it raises the data but it does not update or delete them

<?php header("Access-Control-Allow-Origin: *");

$HOST = 'localhost';
$BASE = 'db';
$USER = 'usr';
$PASS = 'pass';

$conexion = mysqli_connect($HOST, $USER, $PASS,$BASE)
or die("Sin conexión");

$sql = "SELECT * FROM notasGuardadas";

if(!$result = mysqli_query($conexion, $sql)) die();

$consulta = array();

while($row = mysqli_fetch_assoc($result)){
    $consulta[] = $row;
}

$json = json_encode($consulta);
echo $json;

thanks