I want to ask if you can review the code for a simple contact form and the PHP code which sends me an email once a visitor fills the form and is validated through RecaptchaOn my index.php file here are the form and php codeHTML:<form id=contactform action=index.php#contact method=post class=form role=form>    <div class=form-group>      <input class=form-control required id=vname name=vname placeholder=Your Name type=text required />    </div>    <div class=form-group>      <input class=form-control required id=vemail name=vemail placeholder=Your Email type=email required />    </div>    <div class=form-group>      <textarea class=form-control required id=msg name=msg placeholder=Your Message rows=7 required></textarea>    </div>    <div class=form-group>      <div class=g-recaptcha data-sitekey=GOOGLE PUBLIC KEY></div>    </div>    <div class=form-group>      <input type=submit class=btn btn-success form-send value=Send>    </div>  </form>PHP:<?php  $captcha;    if (isset($_POST['g-recaptcha-response'])) { $captcha = $_POST['g-recaptcha-response']; }  // Check for correct reCAPTCHA    $response = file_get_contents(https://www.google.com/recaptcha/api/siteverify?secret=GOOGLE SECRET KEY&response= . $captcha . &remoteip= . $_SERVER['REMOTE_ADDR']);    if (!$captcha || $response.success == false) {        echo Your CAPTCHA response was wrong;        exit ;      } else {        // Check for Blank Fields..        if ($_POST[vname] ==  || $_POST[vemail] ==  || $_POST[msg] == ) {    echo Please fill all required fields;} else {    // Check if the Sender's Email input field is filled out    $email = $_POST['vemail'];    // Sanitize E-mail Address    $email = filter_var($email, FILTER_SANITIZE_EMAIL);    // Validate E-mail Address    $email = filter_var($email, FILTER_VALIDATE_EMAIL);    if (!$email) {        echo Invalid Sender's Email;    } else {        $to = 'myemail@mail.com';        $subject = 'New Form Entry';        $message = New message was submitted from <br />  . <strong> . $_POST['vname'] . </strong> . <br /><br />The message is:<br /> . <strong> . $_POST['msg'] . </strong>;        $headers = From: . $_POST['vname'] . < . $email . >;        $headers .= MIME-Version: 1.0 . \r\n;        $headers .= Content-type:text/html;charset=UTF-8 . \r\n;        // Sender's Email        // Message lines should not exceed 70 characters (PHP rule), so wrap it        $message = wordwrap($message, 70, \r\n);        // Send Mail By PHP Mail Function        if (mail($to, $subject, $message, $headers)) {            echo Your mail has been sent successfully!;        } else {            echo Failed to send email, try again.;            exit ;        }    }}}?>The only function of this is to send me an email and it works as I want it, but I am not aware if this form is secure? Is it possible that someone can exploit this code and upload a shell, or do any other sort of attack against my site.
PHP e-mail form with ReCAPTCHA
php;security;form;email;captcha