accessing a $_SESSION variable is failing

I am trying to build a string to use in this cmd

$cmd = "soffice --convert-to png ".$inputFile." --outdir ". $ouputDirectory;

I tried using $inputFile="/app".{$field_info['upload_path']}; to build the input file name. But I get this error:

Error: Cannot use object of type stdClass as an array

$field_info is coming from the function:

function callback_after_upload2($uploader_response,$field_info, $files_to_upload){}

proof that an old dog can learn new tricks