error checking question

Hello everyone, I've been experimenting with checking for errors in data inserted into a table using an html form. The main page displays the result of a "select * from the_table" at the top of the page, with the form for inserting the data beneath. The SQL for the insert occurs on a different page, with redirection back to the main page using the header() function ( as I've learned to do from reading this list ). This all works well. The inserted data shows up in the table, and there are no surprises when refreshing, etc... I have also implemented some error checking on the client side using JavaScript, and this is also working well. What I would like to do is somehow have a redundant error check on the server side and then display an error message above the form on the main page should fields be left blank or forbidden characters entered. Since I have access to the $_POST['variables'] on the form processing page, I've already written some code to detect the presence of error characters, etc...and no query is performed if there is an error. But how do I pass a "flag" variable back to the main page to echo an appropriate message above the form? Advice or suggestions greatly appreciated. David