Using PHP variable in system() function

I am currently using this command: system('"C:/xampp/htdocs/csv/txtfiles/PSPPfile.txt"');

I want to having something like with having variable inside, for instance:

$file='txtfiles/PSPPfile.txt';
system('"C:/xampp/htdocs/csv/$file"');

Something like above, kindly help me out. Thanks!