try this. in place of username, put administrator username and password. put the domain if exists. then put the name of bat file there. the bat file would contain the shutdown command. also try directly putting the command there.
runas /U username /P password /D Domain /E shutdown.bat
Researched from the community, found this helpful
First up, just to let you know that you're trying to reinvent the
wheel. What you're really looking for is expect(1), which is a
command-line utility intended to do exactly what you want without
involving PHP.
However, if you really want to write your own PHP code you need to use
proc_open. Here are some good code examples on reading from STDOUT and
writing to STDIN of the child process using proc_open:
http://www.php.net/manual/en/function.proc-open.php#79665 How to pass
variables as stdin into command line from PHP
http://camposer-techie.blogspot.com/2010/08/ejecutando-comandos-sobre-un-programa.html
(this one is in Spanish, sorry, but the code is good) Finally, there
is also an Expect PECL module for PHP.
Hope this helps.
source : PHP Exec command - How to pass input to a series of questions