PHP_SELF

Im trying to introduce some logic into a form.I am unsing $PHP_SELF as the target. <form action=\"$PHP_SELF\" method=\"POST\" type=\"multipart/form-data\"> I have a text area that the user inputs their age. What I want is that if the field is blank, to stop the script and send the user back to fill in their age. My problem now is that if I go to the blank application form, The script interprets the form as being blank and spits out the error. Ideal scenario, a blank form is presented to the user, and when the form is submitted, do the logic check and act accordingly. Is it possible using $PHP_SELF as the target, or do I have to use 2 files: a form 'front end' with the php logic in the 'backend'?? if ($Age = " ") {

        echo "Please go back and enter your age."; } exit();

Thanks Gerard