html form send php add checkbox

i have form html and sender.php what add checbox terms in sender.php?

and sender.php what configure sender.php?

<form name="htmlform" method="post" action="html_form_send.php"> 
        <label for="first_name">Nome *</label>
     <input name="first_name" maxlength="50" size="30" type="text"> 
        <label for="last_name">Cognome *</label> 
     <input name="last_name" maxlength="50" size="30" type="text"> 
        <label for="email">Email*</label> 
     <input name="email" maxlength="80" size="30" type="text">
         <p></p>
        <label for="telephone">Telefono</label> 
     <input name="telephone" maxlength="30" size="30" type="text"><br>
        <label for="comments">Richiesta *</label>
    <textarea name="comments"maxlength="1000" cols="32" rows="6"></textarea><br>

        <input name="terms" value="terms" required="required" type="checkbox">
    ho letto l'informativa<br>
    <input value="Invia" type="submit">
</form>

I need to use this php code and not other examples, how can I do? There is a solution?

why can not I put all the php code?

help me please, why do not I use all the code? I think we need to enter all the code, I think

<?php
     if(isset($_POST['email'])) {      
        // CHANGE THE TWO LINES BELOW
        $email_to = "emaimymymytest@gmail.com";      
        $email_subject = "Contatto da Sito pippo";
        function died($error) {
            // your error code can go here
            echo "We are very sorry, but there were error(s) found with the form you submitted. ";
            echo "These errors appear below.<br /><br />";
            echo $error."<br /><br />";
            echo "Please go back and fix these errors.<br /><br />";
            die();
        }
        // validation expected data exists
        if(!isset($_POST['first_name']) ||
            !isset($_POST['last_name']) ||
            !isset($_POST['email']) ||
            !isset($_POST['telephone']) ||
            !isset($_POST['comments'])) {
            died('We are sorry, but there appears to be a problem with the form you submitted.');       
        }
        $first_name = $_POST['first_name']; // required
        $last_name = $_POST['last_name']; // required
        $email_from =  'info@hjbhbhb.it';// required $_POST['email'];
        $telephone = $_POST['telephone']; // not required
        $comments = $_POST['comments']; // required 
        $error_message = "";
        $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
      if(!preg_match($email_exp,$email_from)) {
        $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
      }
        $string_exp = "/^[A-Za-z .'-]+$/";
      if(!preg_match($string_exp,$first_name)) {
        $error_message .= 'The First Name you entered does not appear to be valid.<br />';
      }
      if(!preg_match($string_exp,$last_name)) {
        $error_message .= 'The Last Name you entered does not appear to be valid.<br />';
      }
      if(strlen($comments) < 2) {
        $error_message .= 'The Comments you entered do not appear to be valid.<br />';
      }
      if(strlen($error_message) > 0) {
        died($error_message);
      }
        $email_message = "Form details below.\n\n";
        function clean_string($string) {
          $bad = array("content-type","bcc:","to:","cc:","href");
          return str_replace($bad,"",$string);
        }          
        $email_message .= "First Name: ".clean_string($first_name)."\n";
        $email_message .= "Last Name: ".clean_string($last_name)."\n";
        $email_message .= "Email: ".clean_string($_POST['email'])."\n";
        $email_message .= "Telephone: ".clean_string($telephone)."\n";
        $email_message .= "Comments: ".clean_string($comments)."\n";
    // create email headers
    $headers = 'From: '.$email_from."\r\n".
    'Reply-To: '.$email_from."\r\n" .
    'X-Mailer: PHP/' . phpversion();
    mail($email_to, $email_subject, $email_message, $headers);  
    ?>
    <!-- place your own success html below -->
    Messaggio Inviato...
    <meta http-equiv="refresh" content="3;url=http://www.sitositosito.it/test/pages/thanks.html">
    <?php
    }
    die();
    ?>