<form  action="<?php URL  ?>imagen" method="POST" enctype="multipart/form-data" >
        <input type="file" name="foto" id="boton_enviar">
        <input type="button" name="enviar" value="enviar" onclick="enviarid()">
</form>

function enviarid(){
  var ajax = new XMLHttpRequest()
  id = sessionStorage.getItem("id")
  ajax.open("POST","localhost/imagen",true)
  ajax.send("id="+id);

}

$id = $_POST["id"];
  $img = $_FILES['foto']["name"];
  $save = $_FILES['foto']['tmp_name'];
  $ruta = $img;
  copy($save, $ruta);
  echo "El id: ",$id,"subio una foto";