How to pass microtime(true); (php) in an html form

Question

and then i want to call a php file with a submit button so i can know how many times it took to the person to take the test

 <p class="Text"><font face="Arial"><font face="Arial"><font size="3"
          face="Arial">Question 187&nbsp;: Je suis sûr de n'avoir passé aucune question ni d'avoir omis d'y répondre de façon appropriée.</font></font></font></p>
    <p class="Text"><font face="Arial"><font face="Arial"><font face="Arial"><font size="2"><input type="radio"
           value="<?php $timestart = microtime(true);?>"
           name="choix187" />Oui
    <br /></font></font></font></font></p>
    <p class="Text"><font face="Arial"><font face="Arial"><font face="Arial"><font size="2"><input type="radio"
           value="<?php $timestart = microtime(true);?>"
           name="choix187" />Incertain
    <br /></font></font></font></font></p>
    <p class="Text"><font face="Arial"><font face="Arial"><font face="Arial"><font size="2"><input type="radio"
           value="<?php $timestart = microtime(true);?>"
           name="choix187" />Non
    <br /></font></font></font></font></p>
    <p class="Text">&nbsp;</p><font face="Arial"><font face="Arial">
    <p><input type="submit" name="envoi" value="Envoyer" /></p>


if(isset($_POST['choix186']) ){
    $timestop = microtime(true);
    $time = $timestop - $timestart; 
    }