In the SQL update query, put the field names in ``, also the names such as First Name and Last Name do not look correct, make sure that the names you use in the query are exactly what shows on the database (through phpmyadmin or similar)

The $_POST[] values are fine, they relate to the input fields on the form so that is not a problem.

Also, your SQL update query uses $id which has not been passed to the function from the form, change the form action from "<? echo $PHP_SELF; ?>" to "<? echo $PHP_SELF; ?>?id=$id" and change
to