What's wrong with my PHP code

1

I’m trying to set up a newsletter sign-up form with only on text box ‘email’. If the box is not filled in you are sent to an error page. If it is filled in you are sent to a thank-you page.

Now I’ve got it working but have discovered a problem. Anyone can type in anything they want whether it’s an email address or not. I would like the form.php to recognize the “@” sign before sending, if it’s not there then the error page would appear. Any thoughts on what to change in the code below for this function to work? I haven’t been able to figure it out.

Thanks.


<?

//Declare the variables
$siteaddress ="http://www.default.com"; 
$sitename = "default design"; 
$adminaddress = "newsletter@default.com";
$subject = "SIGN UP";



//Gets date and time from server
$date = date("m/d/Y H:i:s");

// Gets the IP Address
if ($REMOTE_ADDR == "") $ip = "no ip";
else $ip = getHostByAddr($REMOTE_ADDR);

//Contents of form


//////////////////////////////////////////////////////////////////


 $email = $_REQUEST['email'] ;


  if (!isset($_REQUEST['email'])) {
    header( "Location: http://www.default.com/thankyou.htm" );
  }
  elseif (empty($email)) {
    header( "Location: http://www.default.com/error.htm" );
  }
  else {
    
    header( "Location: http://www.default.com/thankyou.htm" );	

/////////////////////////////////////////////////////////////////////
//mail() function sends the mail

	mail ("$adminaddress","Info Request",
	"A visitor at $sitename has left the following information
 
	Email: $email

	The visitor has subscribed to the NEWSLETTER.
	------------------------------

	
	Logged Info :
	------------------------------
	Using: $HTTP_USER_AGENT
	Hostname: $ip
	IP address: $REMOTE_ADDR
	Date/Time:  $date","FROM:$adminaddress" ) ; 



//This sends a confirmation to your visitor
mail ("$email","Thank You for visiting $sitename", 
	"Hi $email,

	Thank you for your interest in $sitename!
 
	You have just subscribed to the default.com newsletter.

	Thanks again!!

	$sitename
	CHANGING THE WORLD!

	$siteaddress
	$adminaddress", "FROM:$adminaddress") ; 

}
//This line sends to thankyou page when finished

?>