How to create a JavaScript variable using PHP

Question

I have a form that sets the time of A timer to something that the user inputs. The form looks like this:

<form action="/setTime.php">
          <center><select id="monthSelectBox" name="monthSelect">
            <option value="" disabled selected hidden>Select A Month&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</option>
            <option value="January">January</option>
            <option value="Feburary">Feburary</option>
            <option value="March">March</option>
            <option value="April">April</option>
            <option value="May">May</option>
            <option value="June">June</option>
            <option value="July">July</option>
            <option value="August">August</option>
            <option value="September">September</option>
            <option value="October">October</option>
            <option value="November">November</option>
            <option value="December">December</option>
            </select></center>
          <center><select id="daySelectBox" name="daySelect">
            <option value="" disabled selected hidden>Select A Day&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</option>
            <option value="1">1</option>
            <option value="2">2</option>
            <option value="3">3</option>
            <option value="4">4</option>
            <option value="5">5</option>
            <option value="6">6</option>
            <option value="7">7</option>
            <option value="8">8</option>
            <option value="9">9</option>
            <option value="10">10</option>
            <option value="11">11</option>
            <option value="12">12</option>
            <option value="13">13</option>
            <option value="14">14</option>
            <option value="15">15</option>
            <option value="16">16</option>
            <option value="17">17</option>
            <option value="18">18</option>
            <option value="19">19</option>
            <option value="20">20</option>
            <option value="21">21</option>
            <option value="22">22</option>
            <option value="23">23</option>
            <option value="24">24</option>
            <option value="25">25</option>
            <option value="26">26</option>
            <option value="27">27</option>
            <option value="28">28</option>
            <option value="29">29</option>
            <option value="30">30</option>
            <option value="31">31</option>
            </select></center><br>
            <p align="center">Set the Time Here:</p>
            <center><input style="display: inline;" id="timeInputBoxHr" pattern="[0-9]" size="1" maxlength="2" placeholder="hr:" oninvalid="Please correct this field!" required>:<input style="display: inline;" id="timeInputBoxMin" pattern="[0-9]" size="1" maxlength="2" placeholder="min:" oninvalid="Please correct this field!" required>:<input style="display: inline;" id="timeInputBoxSec" pattern="[0-9]" size="1" maxlength="2" placeholder="sec:" oninvalid="Please correct this field!" required>
            <select id="AMorPM" style="padding: 0.5em 3.5em 0.5em 1em;" required>
              <option value="" disabled selected hidden>AM/PM?</option>
              <option value="AM">AM</option>
              <option value="PM">PM</option>
            </select></center><br><br>
          <center><button>Submit</button></center>
          </form>

I want the php file to set a variable in javascript to set it as the time variables in the timer. How can I do this? I have never used PHP before.