code: function getSelectorValue(page, ph, selector, callback) {
      page.evaluate(function (selector) {
          try {
              return [null, document.querySelector(selector).value];
          } catch (error) {
              return [error];
          }
      }, function (result) {
          var error = result[0];
          var selectorValue = result[1];
          if (error) {
              callback(error, page, ph);
              return;
          }
          callback(null, page, ph, selectorValue);
      }, selector);
  }
docs: Gets the .value of a selector
  @param page
  @param ph
  @param selector
  @param {pageResultCallback} callback