"" and "_blank" gives the same problem, it looks like it depends on php print function and how php script is called, this seems to work:
in Flash:
//this will be used to get variable back from php script
recVars.txt = "";
//I don't know what this is for
var result_lv:LoadVars = new LoadVars();
gatherForm.sendAndLoad("send_mail.php", result_lv, "POST");
in PHP:
<?php
//create short variable names
$name=$_POST['visitor_name'];
$email=$_POST['visitor_email'];
$subject=$_POST['visitor_subject'];
$message=$_POST['visitor_comments'];
$name=trim($name);
$email=trim($email);
$subject=StripSlashes($subject);
//lets make the message more explaining
$message="Name: $name\nEmail: $email\nComments: " . StripSlashes($message);
//email address where to mail the content of form to
$toaddress=$_POST['email_to'];
mail($toaddress,$subject,$message,"From: $name <$email>");
//clear the variables
$name='';
$email='';
$subject='';
$message='';
//echo "I don't want this page open!!!!";
//now it si sending back variable recVars to flash, this will prevent opening the separate browser window
print "recVars=OK";
?>