How do I send emails from a .php file? I have developed a separate functionality using PHP in a sub folder of my Drupal site (e.g. mysite.com/myfolder/myfunc.php). 

Now I want to send emails using Drupal functions. Since this is not a custom module I can't use hook_mail(). 

How do I send an email through Drupal?
Try like this:

$to = 'ABCD@example.com'; 
$subject = 'Email in PHP'; 
$message = 'This is a test email message';
$headers = 'From: ABCD@example.com' . "\r\n" . 'Reply-To: webmaster@example.com' . "\r\n" . 'X-Mailer: PHP/' . phpversion(); 
mail($to, $subject, $message, $headers, 'ABCD@example.com');