PHP在文件下载中使用变量

I have a variable generated by a command that is ran. Everything seems to work fine and I get a file download dialog with the correct filesize. Issue is I am unable to get my varible to be the actual filename. It is always $fullpath as my filename

I am have tried many different ways including without quotes

 header('Content-Type: application/download');
    header('Content-Disposition: attachment; filename="$fullpath"');
    header("Content-Length: " . filesize("$fullpath"));

    $fp = fopen("$fullpath", "r");
    fpassthru($fp);
    fclose($fp);

PHP doesn't parse variables in single quotes '. Use double quotes " but you also need to escape any double quotes you want as part of the string by using \".

header("Content-Disposition: attachment; filename=\"$fullpath\"");

or concat the variable and the string.

header('Content-Disposition: attachment; filename="' . $fullpath . '"');

Also, adding quotes in these places serves no purpose:

filesize($fullpath);
fopen($fullpath);