Populate field depending on $_POST value

I am looking for some help with a form I am trying to populate.

I would usually use an onChange() function, but I don't think that is an option here.

Basically I have a with a couple of hidden values, and a submit button. When submit is hit, it directs to the booking page, where the $_POST values populate a couple of the fields.

<form method="post" action="book.php">
    <input type="hidden" name="title" value="Cardio Fitness">
    <input type="hidden" name="courseID" value="CF">
    <input type="hidden" name="day" value="Wednesday">
    <input type="hidden" name="time" value="9:00pm">
    <input type="submit" Value="Wednesday - 9:00pm">
</form>

The book.php looks like:

<h2><?php echo $_POST['title'];?></h2>

<form id="booking" onSubmit="return check_book()" method="post" action="#">
    <!-- Course Name -->
    <input type="hidden" name="courseID" id="cID" value="<?php echo $_POST['courseID'];?>"/>
    <!-- Course Day -->
    <input type="text" name="day" id="day" value="<?php echo $_POST['day'];?>" readonly/>
    <!-- Course Time -->
    <input type="text" name="time" id="time" value="<?php echo $_POST['time'];?>" readonly/>

    <!-- Adult Spots -->
    <select name="as" class="1-10">
        <option value="" disabled selected>Adult</option>
    </select>
    <input type="text" class="as_price" readonly/>
    <!-- Child Spots -->
    <select name="cs" class="1-10">
        <option value="" disabled selected>Child</option>
    </select>
    <input type="text" class="cs_price" readonly/>

    <!--Submit Button -->
    <input type="submit" value="Add to Cart"/>

</form>

What is the best way to populate the .as_price and .cs_price fields, depending on the $_POST values? JS?

Am I completely off the mark?

Cheers