> ls
F001.php F002.php F004.php F006.php F008.php
F0010.php F003.php F005.php F007.php F009.php
for i in `ls`; do mv -f $i `echo $i | sed 's/php/txt/'`; done
> ls
F001.txt F002.txt F004.txt F006.txt F008.txt
F0010.txt F003.txt F005.txt F007.txt F009.txt