File download problem

[pzbrawl]

This code, run from a shared linux/apache web server, fails at the end of the download with a "source file could not be read" error at the client.
[code=php:0]
    $path = "xxx/";
    $file = "yyy.zip";
    $filetype = "application/x-zip-compressed";
    $fsize = filesize( $path . $file );
    header( "Content-Disposition: attachment; filename=\"$file\"");
    header( 'Content-type: '. $filetype );
    header("Content-Length: $fsize" );
    header( 'Pragma: no-cache' );
    header( 'Expires: 0' );
    return @readfile( $path . $file );
[/code]
When the content-length header is not sent, some users can download successfully while others get incomplete zips.

Setting type to "application/octet-stream" or "encoding/x-compress" yields the same result.

Setting type to "application/gzip" gets rid of the read error when length is set but still yields an invalid zip.

Could anyone please suggest what might be going on?