html - not receiving emails from php mail() function

I have recently been learning HTML, js, CSS, and PHP through creating web pages and posting them on my apache2 server hosted on my raspberry pi with a no-ip.com address. My first real project for learning PHP has been creating a simple form to then send an email to me, but I keep running into the issue of the mail() function executing without any errors, but not actually sending emails.

Here is the form:

  Name:

  Email:

  Message:

  Subject:
  Hello!
  Whats Up!
  

Here is the PHP file reference in the form's 'action' attribute:

if($_SERVER["REQUEST_METHOD"] == "POST") {
  $to = "myExampleEmail@gmail.com";
  $name = $_POST["name"];
  $email = $_POST["email"];
  $message = $_POST["message"];
  $subject = $_POST["subject"];


  echo $name;
  echo $email;
  echo $message;
  echo $subject;

  $headers = array("From: " . $email,
"Reply-To: " . $to,
"X-Mailer: PHP/" . PHP_VERSION);


  mail($to, $subject, $message, $header);

} else {
  echo $_SERVER["REQUEST_METHOD"];
}
?>

I understand that this question has been asked before, and I have tried most of the methods set out by other answers, but I still can't seem to get the PHP mail() function to work. The best information I have is that I have to do some kind of install or formatting on the server itself to allow for emails to be sent, but I have yet to find it well documented and/or explained. (i.e phpMailer? - but still... how?)

EDIT:

I checked the return value of the mail function . . .

  if(mail("myExampleEmail@gmail.com", "subject", "message", $headers)) {
    echo "TRUE";
  }else {
    echo "FALSE";
  } 

. . . and it turns out to be returning false. I assume that means it is an error in my code, then?

EDIT:

Apparently I wasn't clear when I said I did research before posting this question. I tried everything that was given as an answer to the question that mine was a closed as a "duplicate" of, hence me asking, the question anyway.