How to copy a file from one directory to another with PHP?

I have 2 websites on the same linux server, but as is logical, there are different folders, with this code I can copy files within the same domain so to speak. How can I copy a file from one domain to another? and in case I have to use ftp, I can only use sftp.

$fichero = 'ejemplo.txt';
$nuevo_fichero = 'carpeta/ejemplo.txt'; // esta es la carpeta dentro del mismo dominio
//me gustaria poder escribir algo como: /var/www/html/carpetadestino/ejemplo.txt

if (!copy($fichero, $nuevo_fichero)) {
    echo "Error al copiar $fichero...\n";
}