Meaning of error?

Hello; I have been getting this error: <b>Warning</b>: Illegal string offset 'file' in <b>(path)/dev_lab_current/php/console_proc.php</b> on line <b>69</b><br /> This error, with the markup, is appearing the the top of the page that is supposed to display processing results. The line number represents this code: if(!self::$_input['file']) Why would 'file' be an illegal string offset? Is it literally illegal or is there some other reason for this error? This is the first time I have seen this. Thanks for time and attention: JK