PHP - Php Validation Form

Hi. I'm new to PHP and have a problem with some form validation.

The code below is my contact form. It was working until I added a dropdown list and checkbox. For all the other fields I created an input variable using the 'name' attribute to reference the data entered into the input fields and the validation appeared to work and an email was sent to the appropriate email address.

For the checkbox field I have also created a 'name' attribute but don't know whether this is right way to confirm whether the box has been checked?

For the field with a dropdown selection I have used the option 'selected', but again, don't know whether this is right or not.

Now when I enter valid data into all the fields, select a value from the dropdown and check the checkbox, I get an error message stating that I need to fill in all the form fields. I am pretty sure it is to do with the way I have tried to pick up the data from dropdown list and the checkbox, but can't figure out what it is. 

Any help would be much appreciated.

Thanks in advance.

<?php echo file_get_contents('header.php'); ?>

<?php
    // Message Vars
    $msg = '';
    $msgClass = '';

  // Check to see if the form has been submitted
  if(filter_has_var(INPUT_POST, 'submit')) {
    //Input Data Variables
    $firstname = htmlspecialchars($_POST['firstname']);
    $lastname = htmlspecialchars($_POST['lastname']);
    $phonenumber = htmlspecialchars($_POST['phonenumber']);
    $email = htmlspecialchars($_POST['email']);
    $selected = htmlspecialchars($_POST['selected']);
    $message = htmlspecialchars($_POST['message']);
    $checkbox = htmlspecialchars($_POST['checkbox']);


    //Check required fields
    if(!empty($firstname) && !empty($lastname) && !empty($phonenumber) && !empty($email) && !empty($selected) && !empty($message) && !empty($checkbox)) {
      // If passed, check email address
      if(filter_var($email, FILTER_VALIDATE_EMAIL) === false) { 
        //If failed, Email address is not valid
        $msg = 'Please use a valid email address';
        $msgClass = 'alert-danger';
      } else {
        //Recipient email address and information to be sent to them
        $toEmail = 'hello@example.co.uk';
        $subject = 'Contact Request from  Website';
        $body = '<h2>Contact Request</h2>
                <h4>Name</h4><p>'.$firstname. ''.$lastname.'</p>
                <h4>Phone Number</h4><p>'.$phonenumber.'</p>
                <h4>Email Address</h4><p>'.$email.'</p>
                <h4>Subject</h4><p>'.$selected.'</p>
        ';

        // Email Headers
        $headers = "MIME-VERSION: 1.0" ."\r\n";
        $headers .="Content-Type:text/html;charset=UTF-8" ."\r\n";

        // Additional Headers
        $headers .= "From: " .$firstname. " ".$lastname." <".$email.">" ."\r\n"; 

        if(mail($toEmail, $subject, $body, $headers)) {
          // Message Sent
          $msg = 'Your message has been sent';
          $msgClass = 'alert-success';
        } else {
          // Message failed
          $msg = 'Your message has NOT been sent';
          $msgClass = 'alert-danger';
        }
      }  
    } else {
      //Failed
      $msg = 'Please fill in all fields';
      $msgClass = 'alert-danger';
    }
  }


?>



      <div class="container clearfix content-container">
        <h1 class="section-title">Contact Us</h1>
        <p>If you have any questions about the services we provide or would like to chat about a new website project, please get in touch using any of the options below. We would love to hear from you!</p>

        <!--Contact Page Row-->
        <div class="row">
          <div class="col-lg-6 contact-details">
            <div class="row">
              <div class="col-lg-12">
                <img src="assets/img/contact-us.jpg" class="img-responsive d-block contact-image" alt="Contact Us Image">
              </div>
            </div>
            <div class="row">
              <div class="col-lg-12 contact-name">
                <span class="d-inline-block contact-inline-block"><i class="fas fa-user fa-2x">
                </i></span>
                <span class="d-inline-block"><p class="contact-details">sn0wman23</p></span>   
              </div>
            </div>
            <div class="row">
              <div class="col-lg-12 contact-phone">
                <span class="d-inline-block contact-inline-block"><a href="tel:+447740484798"><i class="fas fa-mobile-alt fa-2x"></i></a></span>
                <span class="d-inline-block"><a href="tel:+441234567890"><p class="contact-details">01234 567890</a></p></span>  
              </div>
            </div>
            <div class="row">
              <div class="col-lg-12 contact-email">
                <span class="d-inline-block contact-inline-block"><a href="mailto:hello@example.co.uk"><i class="fas fa-envelope-square fa-2x"></i></a></span>
                <span class="d-inline-block"><a href="mailto:hello@example.co.uk"><p class="contact-details">hello@example.co.uk</a></p></span>  
              </div>
            </div>
            <div class="row">
              <div class="col-lg-12 contact-fb">
                <span class="d-inline-block contact-inline-block"><a href="https://www.facebook.com/" target="_blank"><i class="fab fa-facebook-square fa-2x"></a></i></span>
                <span class="d-inline-block"><a href="https://www.facebook.com/example/" target="_blank"><p class="contact-details">@example</a></p></span> 
              </div>
            </div>
          </div>

          <!--Contact Form-->
          <div class="col-lg-6 contact-form d-block">
            <?php if($msg != ''): ?>
              <div class="alert <?php echo $msgClass; ?>"><?php echo $msg; ?></div>
            <?php endif;?>
            <form method="post" action="contact.php" role="form">
              <div class=" row form-group">
                <div class="col-lg-6">
                  <label for="firstnameid">First name:</label>
                  <input type="text" name="firstname" class="form-control mb-3" id="firstnameid">
                </div>
                <div class="col-lg-6">
                  <label for="lastnameid">Last name:</label>
                  <input type="text" name="lastname" class="form-control mb-3" id="lastnameid">
                </div>
              </div>
              <div class="row form-group">
                <div class="col-lg-6">
                  <label for="phonenumber">Phone number:</label>
                  <input type="tel" name="phonenumber" class="form-control mb-3" id="phonenumberid">
                </div>
                <div class="col-lg-6">
                  <label for="emailid">Email address:</label>
                  <input type="email" name="email" class="form-control mb-3" id="emailid">
                </div>
              </div>
              <div class="row form-group">
                <div class="col-lg-12">
                  <label for="subjectid">How can I help?:</label>
                  <select class="form-control mb-3" id="subjectid">
                    <option selected>Select one from this list</option>
                    <option value="1">Value 1</option>
                    <option value="2">Value 2</option>
                    <option value="3">Value 3</option>
                  </select>
                </div>
              </div>
              <div class="row form-group">
                <div class="col-lg-12">
                  <label for="messageid">Tell me a little bit mo </label>
                  <textarea name="message" class="form-control mb-3" id="messageid" rows="6"></textarea>
                </div>
              </div>
              <div class="row form-group">
                <div class="col-lg-12 form-check">
                  <input type="checkbox" name="checkbox" class="form-check-input" id="formcheckid">
                  <label for="formcheckid" class=form-check-label mb-3>By checking this tickbox you have confirmed that we can collect the information in this form for the purposes outlined in our <a href="privacy-policy.html">privacy policy.</a></label>
                </div>
              </div>
              <div class="row form-group">
                <div class="col-lg-12">
                  <button type="submit" name="submit" class="btn btn-primary mt-4">Send Message
                  </button>
                </div>
              </div>
            </form>
          </div>
          <!--Contact Form End-->
        </div>
        <!--Contact Page Row End-->
      </div>
      <!--Container end-->

<?php echo file_get_contents('footer.php'); ?>

Hello All,

I am a newbie to PHP and I'm working on a marathon sign up form for my final project and could really use the help. The trouble i'm having is sending and retrieving database from the database. Below is the code i am using. Can someone please review and tell me where I went wrong.

(This is the code that is sitting at the top of my document)

<?php
session_start();
require_once('inc/dbuser.php');
function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}
if (isset($_POST['submit_registration'])) {
$username = test_input($_POST['username']);
$password = test_input($_POST['password']);
$cpassword = test_input($_POST['cpassword']);
$fname = test_input($_POST['fname']);
$lname = test_input($_POST['lname']);
$email = test_input($_POST['email']);
$phone = test_input($_POST['phone']);
$address = test_input($_POST['address']);
$city = test_input($_POST['city']);
$state = test_input($_POST['state']);
$zip = test_input($_POST['zip']);
$distance = test_input($_POST['distance']);
if (!empty($username) || !empty($password) || !empty($fname) || !empty($lname)|| !empty($email) || !empty($phone)|| !empty($address)|| !empty($city)|| !empty($state)|| !empty($zip)|| !empty($distance)) {

}
else{
    
     $sql = "SELECT email From runners Where email = ? Limit 1";
     //Prepare statement
     $stmt = mysqli_stmt_init($conn);
     if(!mysqli_stmt_prepare($stmt, $sql)){
       header("location: ../signup?error=sqlerror");
      exit();  
     }
     else{
     mysqli_stmt_bind_param($stmt,'s', $email);
     mysqli_stmt_execute($stmt);
     $stmt->bind_result($email);
     $stmt->store_result($stmt);
     $resultCheck = mysqli_stmt_num_rows($stmt);
     if ($resultsCheck > 0) {
      header ("Location: ../signup.php?error=usertaken&email=".$email);
      exit();
     }
     else {
         $sql = "INSERT Into runners (username, password, fname, lname, email, phone, address, city, state, zip, distance) values(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
         $stmt = mysqli_stmt_init($conn);
         if(!mysqli_stmt_prepare($stmt, $sql)) {
             header ("Location: ../signup.php?error=sqlerror");
      exit();
         }
         else{
            $hashedPwd = password_hash($password, PASSWORD_DEFAULT);
         mysqli_stmt_bind_param($stmt,"ssssssssssss", $username, $hashedPwd, $fname, $lname, $email, $phone, $address,$city, $state, $zip, $distance);
         mysqli_stmt_execute($stmt);
         header ("Location: ../signup.php?signup=success");
      exit();
         
 }
}
     }    
}
 mysqli_stmt_close($stmt);
     mysqli_stmt_close($conn);
}  
   
?>

(This is my form [submit_registration] )

<div class="contact">
                <?php
                if(isset($_GET['error'])) {
                    if($_GET['error']=="emptyfields"){
                       echo '<p>Fill in all fields!</p>';
                    }
                    elseif($_GET['error']== "invalidemail") {
                       echo '<p>Provide a valid email!</p>';  
                    }
                    elseif($_GET['error']== "usertaken") {
                       echo '<p>Username is already taken!</p>';
                    }
                     elseif($_GET['error']== "passwordCheck") {
                       echo '<p>Your passwords do not match!</p>';
                     }
                      elseif($_GET['signup']== "success") {
                       echo '<p>Signup Successful!</p>';
                      }
                }
                ?>
            <form name="form1" action="signup.php" method="post">
                            <p><label>Create a username:<br></label>
                                    <input type="text" name="username" placeholder="username" autofocus required><br>
                                    <span class="err" id="usrErr"></span>
                                </p>
                                <p><label>Create a Password:<br></label>
                                    <input type="password" name="password" placeholder="password" autofocus required><br>
                                    <span class="err" id="pwdErr"></span>
                                </p>
                                <p><label>Confirm Password:<br></label>
                                    <input type="password" name="cpassword" placeholder="confirm password" autofocus required><br>
                                    <span class="err" id="cpwdErr"></span>
                                </p>
                                <p><label>First Name:<br></label>
                                    <input type="text" name="fname" placeholder="first name" autofocus required><br>
                                    <span class="err" id="fnErr"></span>
                                </p>
                                <p><label>Last Name:<br></label>
                                    <input type="text" name="lname" placeholder="last name" autofocus required><br>
                                    <span class="err" id="lnErr"></span>
                                </p>
                                <p><label>Email:<br></label>
                                    <input type="text" name="email" placeholder="you@youremail.com" required><br>
                                    <span class="err" id="emErr"></span>
                                </p>
                                <p><label>Phone: <br></label>
                                    <input type="tel" name="phone1" placeholder="XXX-XXX-XXXX" maxlength="13" required>
                                    <span class="err" id="phErr"></span>
                                </p>
                                <p><label>Address: <br></label>
                                    <input type="text" name="address" placeholder="415 W. 15th Street" required>
                                    <span class="err" id="addErr"></span>
                                </p>
                                <p><label>City: <br></label>
                                    <input type="text" name="city" placeholder="Chicago" required>
                                    <span class="err" id="cityErr"></span>
                                </p>
                                <p><label>State:<br></label>
                                    <input type="text" name="state" placeholder="IL" maxlength="2" required>
                                    <span class="err" id="stateErr"></span>
                                </p>
                                <p><label>Zip: <br></label>
                                    <input type="text" name="zip" placeholder="60621" maxlength="5" required>
                                    <span class="err" id="zipErr"></span>
                                </p>
                                <p>I am interested in:<br>
                                <span id="inErr" class="err"></span>
                                <select id="interest" class="adj">
                                    <option disabled>--5k--</option>
                                    <option> 1 Mile Run</option>
                                    <option> 5K Run</option>
                                    <option>10K run</option>
                                </select>
                                </p>
                                  <button type="submit" name="submit_registration"onclick="validForm()">Signup</button>
 
                                  
                                </p>
                            </form>