The mysqli_query
you are using to check if the email exists needs a reference to the mysql connection.
Change this line:
//query to look up for the email
$query = mysqli_query("SELECT `email` FROM `workshops` WHERE `email` = '$email'");
to this:
//query to look up for the email
$query = mysqli_query($con, "SELECT `email` FROM `workshops` WHERE `email` = '$email'");