Ok, firstly, you should always use the $_POST superglobal when passing form data.
Secondly, your error messages need to be concatenated, so that if any of them are displayed, you see all of them. As it is at the moment, each time $message is called it will overwrite the previous $message variable. Also you don`t need if else statements as message will only ever hold one value. In this instance, just if statements will work best.
And finally, you appear to be missing a brace in your code.
Give this a try:
<?php
$sendto="[email protected]";
$emailsubject="Get Together Attendance";
$thankyou="Your reservation has been submitted. See you in August!";
if ($_POST['submit']) {
if ($_POST['name'] == "")
$message .= "Please enter your name.<br>";
if ($_POST['screenname'] == "")
$message .= "Please enter your GA Screenname.<br>";
if ($_POST['address'] == "")
$message .= "Please enter your address.<br>";
if ($_POST['citystate'] == "")
$message .= "Please enter your city and state.<br>";
if ($_POST['phone'] == "")
$message .= "Please enter your contact phone number.<br>";
if ($_POST['email'] == "")
$message .= "Please enter a valid email address.<br>";
if ($_POST['numberofgliders'] == "")
$message .= "Please enter a number of gliders.<br>";
if ($_POST['numberofadults'] == "");
$message .= "Please enter the number of adults attending.<br>";
if ($_POST['numberofchild'] == "");
$message .= "Please enter the number of children attending.<br>";
if ($message)
{
echo ($message);
exit;
} else {
mail("$sendto",
"$emailsubject",
"rnName: $_POST['name']rnEmail: $_POST['email']rnScreenname: $_POST['screenname']rnAddress: $_POST['address']rnCity/State: $_POST['citystate']rnPhone Number: $_POST['phone']rnDays Attending: $_POST['friday'] $_POST['saturday'] $_POST['sunday'] $_POST['other']rnOther Comments: rn$_POST['otherdaysattending']rnNumber of Gliders: $_POST['numberofgliders']rnNumber of Adults: $_POST['numberofadults']rnNumber of Children: $_POST['numberofchild']rnStaying in Hotel: $_POST['hotel']rnAmusement Park: $_POST['amusementpark']rnGift Exchange: $_POST['giftexchange']rnPlanning Committee: $_POST['planningcommittee']",
"From: $_POST['name'] <$_POST['email']>");
echo ("<br><br> $thankyou");
exit;
}
}
?>
See if that works.