How to make competent validation on forms? Empty letters constantly come!?

Question

PHP

Hi all!
Guys tell me how to make the correct validation on the forms.
no matter what I do, empty letters still come
here is the code that I use at the moment
thanks in advance

<form method="post" action="submit.php" data-abide id="cont-form">
  <div data-abide-error class="alert callout" style="display: none;">
    <p><i class="fi-alert"></i> There are some errors in your form.</p></div>
      <input form="cont-form" type="text" name="yourname" placeholder="Your Name" required>
      <input form="cont-form" type="email" name="email" placeholder="Your Email Address" required>
        <span class="form-error">Your Email Address Is Invalid</span>
        <input form="cont-form" type="text" name="phone" placeholder="Your Phone Number" required >
        <span class="form-error">Please Enter A Valid Phone Number</span>
        <input form="cont-form" type="text" name="hear" placeholder="How Did You Hear About Us?" required>
        <textarea form="cont-form" placeholder="Your Message" type="text" name="message" required></textarea>
                                           			        
<label>What's 13+14 = </label><input form="cont-form" name="answer" type="text" />
              
        <button  type="submit" name="submit" value="submit message" class="button" data-text="submit message"><span>submit message</span></button>
        <div class="clear"></div>
      </form>

$answer = 27;
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
     if ($answer != $_POST['answer'] ) {
    $hasError = true;
    }else{
     $content = '';
if($_POST["yourname"]){
$yourname = $_POST["yourname"];
}

if($_POST["email"]){
   $email=$_POST["email"];  
}

if($_POST["phone"]){
   $phone=$_POST["phone"];   
}

if(!empty($_POST["type"])){
    $type=$_POST["type"];
}
if($_POST["hear"]){
   $hear=$_POST["hear"];
}
if($_POST["message"]){
   $message=$_POST["message"];
    }
}
$content = "Name : " . $yourname . "<br>";
$content .= "Email : " . $email . "<br>";
$content .= "Phone : " . $phone . "<br>";
$content .= "Message : ". $message ."<br>";
$content .= "How Did You Hear? : ". $hear ."<br>";

if($hasError != true){
mail($sendToEmail,$subject,$content,$senderEmailId);
header("Location:thankyou.php");
  exit();
 }
else
 { 
 ?>
 <script language="javascript">
  alert("Sorry,An Error Occurred.Please Try After Some Time");
  window.history.back(); 
 </script>
 <?php
 }

Answer 1

What does error_reporting(E_ALL); show?

Answer 2

1. "13+14" is an undercaptcha, bots easily enter 27 in the answer field and your script will skip the data without asking. That is, you will receive an email, but there will be no values ​​in it. Use normal captcha.
2. I would recommend rewriting the data processing logic from the form: required on the client for inputs, of course, it’s good, but you also need to check the data from the server side, a la

$error = FALSE;
// ... какая-то логика
if(isset($_POST['email']) && filter_var($_POST['email'], FILTER_VALIDATE_EMAIL) !== FALSE) {
   $email = $_POST["email"];
} else 
   $error = 'emailError'; // или текст ошибки или просто TRUE, если нет нужды уточнять
// ... другая логика

if($error === FALSE)
{
   mail($sendToEmail, $subject, $content, $senderEmailId);
   header("Location: thankyou.php");
}
// ... снова логика

This is a very simplified version.