SQL error, not a PHP one. I'll move it to the mysql forum.
$insertData = "INSERT INTO `test` (title,author,content) VALUES ('$title', '$author', '$content')";
Those variables look to be strings. Therefore, you must wrap them in quotations to be treated as strings, otherwise DBMS will attempt to cast them to a double and fail.
Make sure you're properly escaping you're input data with mysql_real_escape_strings, and stripslashing any of the magic_quotes that may or may not be added.