cambiando questa riga da così
exec(sprintf('mysqldump --opt -h %s -u %s -p%s %s | gzip > %s/%s/%s-%s%s%s-%s%s.ql',$host,$user,$pass,$db.' '.$tables,getenv('DOCUMENT_ROOT'),$backupdir,$nome,$year,$month,$day,$hour,$min ));
a così
exec(sprintf('mysqldump --opt -h %s -u %s -p%s %s | gzip > %s/%s/%s-%s%s%s-%s%s.sql.gzip',$host,$user,$pass,$db.' '.$tables,getenv('DOCUMENT_ROOT'),$backupdir,$nome,$year,$month,$day,$hour,$min ));
funziona