Append form input values with some text

If you don't have access to the server side script you'll need to intercept the form submission, hide the form (or use a fake form and a hidden form for the real values), prepend the data and then submit the form. The general idea is you don't want to add the data to the actual form elements during submission because the user will actually see that data appear.

Here's a JSFiddle example of a single visible and hidden input. The value the server side will want is the hidden and I'm only demonstrating this version because I don't have to make the form disappear. I'm not saying this is the best method of doing it, especially if you have a lot of form data. You don't want to duplicate every field. You can just hide the form, show a nice "processing" animation and actually change the form values directly. It would use the same code as this example: Edit fiddle - JSFiddle

After I toss in 11/22/3333 you can see from the element panel the hidden element has the title values prepended:

If you're using PHP to process the form, the best way to handle this is to use descriptive values for the name attribute. The name and ID attributes don't need to be the same (in fact, they can't be with radio button groups).

<form method="post" action="https://shopping_cart/ID">

<label for="arrival">Arrival Date:</label>

<input name="arrival_date" id="arrival" type="text" value="" required placeholder="mm/dd/yyyy">

<label for="nights># of Nights:</label>

<input name="#_of_nights" id="nights" value="" type="number" min="1" required placeholder="How many nights?">

<label for="guests"># of Guests:</label>

<input name="#_of_guests" id="guests" value="" type="number" min="1" required placeholder="How many in your group?">

<button type="submit">ADD TO CART</button>

</form>

In the processing script, create the label using str_replace() and ucfirst():

Alternatively, keep the short name attributes, but use an array for the labels:

$labels = [
    'arrival' => 'Arrival date: '.
    'nights' => '# of nights; ',
    'guests' => '# of guests: '
];