<?php
$dataInicial = "18/10/2017";
$dias = 7;
// Criar o objeto representando a data
$obj_data = DateTime::createFromFormat('d/m/Y', $dataInicial);
$obj_data->setTime(0, 0, 0);
$intervalo = new DateInterval("P{$dias}D");
$obj_data->add($intervalo);
// Formatar a data obtida
echo $obj_data->format('d/m/Y');
PD9waHAKCiRkYXRhSW5pY2lhbCA9ICIxOC8xMC8yMDE3IjsKJGRpYXMgICAgICAgID0gNzsKCi8vIENyaWFyIG8gb2JqZXRvIHJlcHJlc2VudGFuZG8gYSBkYXRhCiRvYmpfZGF0YSA9IERhdGVUaW1lOjpjcmVhdGVGcm9tRm9ybWF0KCdkL20vWScsICRkYXRhSW5pY2lhbCk7CiRvYmpfZGF0YS0+c2V0VGltZSgwLCAwLCAwKTsKCiRpbnRlcnZhbG8gPSBuZXcgRGF0ZUludGVydmFsKCJQeyRkaWFzfUQiKTsKJG9ial9kYXRhLT5hZGQoJGludGVydmFsbyk7CgovLyBGb3JtYXRhciBhIGRhdGEgb2J0aWRhCmVjaG8gJG9ial9kYXRhLT5mb3JtYXQoJ2QvbS9ZJyk7