How can i use a submit button inside a php variable to store to the db?

i am trying to store a variable in database when the user clicks the submit button. However as i my submit button is inside a php variable when i am using isset doesnt work. is there another solution or a trick to that?

my code follows:

$pp_checkout_btn .= '<form id="paypal" action="http://ift.tt/rDmnwQ" method="post">
    <input name="terms"  type="checkbox" required value="agree" form="paypal"  /><label>Agree with terms & Conditions *</label>
    <input type="submit" name="submit1" id="changebutton" value="ΕΠΙΒΕΒΑΙΩΣΗ ΠΑΡΑΓΓΕΛΙΑΣ"  /></form>';

 if(isset("submit1")){
//update orders to accept terms
$updatesql=mysql_query("UPDATE orders SET terms='agree' WHERE checkout_id='$checkout_id'") or die(mysql_error());  }

Any help will be much appreciated, Thank You.