help needed with forms

Hello I am using php based forms, which collect information from the client and send it in the form a email message to the admin. I need to start using validation i.e. check whether some field/s has been correctly entered in the from. My application consists of 2 php pages

1).  info.php    (this page consists of the plain form and the has its action parameter set to "send_info.php")
2).  send_info.php  (this forms collects the information that the client entered and composes a message and mails it to
       admin)

So I use headers in the following manner: The send_info.php code starts like this <? if($email =='') {

    header("Location: http://www.mysite.com/info.php");

} ?> This takes the user back to the info.php page with all the fields reset. What I need is that 1). An alert box must be displayed which tells the user where he went wrong 2). Also the data that the user had entered on the form must not be reset to blank. I know this sounds very trivial. Please help me out. Thanks --pS Pushpinder Singh Garcha