Obtaining PHP variable from HTML

Hello Stackoverflow community,

This is my first site with PHP and I'm making contact form on it that sends mail to me using PHP. I want to set two different messages: 1. If all fields are filled; 2. If one of two specific <input type="text"> are empty , show other message in my mail.

NOTE: If you're getting hard time reading full code, look for 'specific-text-1' and 'specific-text-2'

This is how my HTML [contact.html] looks like:

    <form id="query-form" class="query-form" method="post">
        <input type="text" name="specific-text-1" placeholder="Text 1">
        <br>
        <input type="text" name="specific-text-2" placeholder="Text 2">
        <br>
        <input type="text" name="subject" placeholder="Email Subject">
        <br>
        <textarea name="message" placeholder="Message text" maxlength = "700"></textarea>
        <br>
        <input type="submit" name="submit" placeholder="Send Message">
    </form>

In other file [contact.php], this is my code:

<?php echo file_get_contents("html/support.html"); 
  if (isset($_POST['submit'])) {
  $subject = $_POST['subject'];
  $message = $_POST['message'];
  $specific1 = $_POST['specific-text-1']; // THIS IS TEXT I WANT TO USE IF 
  $specific2 = $_POST['specific-text-2']; // THIS IS TEXT I WANT TO USE IF 
  $mailTo = "mail@mymail.com";

  $txt = "Message received.\n\nSpecific-text-1: ".$specific1."\n"."Specific-text-2: ".$specific2."\n\n"."Message context: \n\n".$message;
  mail($mailTo, $subject, $txt);
  }
?>

instead of $txt = "Message received...", I want to use:

if (Specific-text-1 !== '' || Specific-text-2 !== '') {
  $txt = "Message2 received.\n\n"."Message context: \n\n".$message;
}
else {
  $txt = "Message received.\n\nSpecific-text-1: ".$specific1."\n"."Specific-text-2: ".$specific2."\n\n"."Message context: \n\n".$message;
}

My question is, how do I get PHP to know if specific-text-1 or specific-text-2 texts that user types are empty and send Message2 or if are both full send Message?

Sorry if I couldn't find answer but there is such, I'm really new to PHP and Javascript. Thank you all very much!