usr is static. It is same for all the servers. It is not your username but a default folder set up by DH.
If you are not sure about PHP path, you can type command "which php".
If you setup your own PHP, you should give the path of your own PHP like /home/username/php/php (sample)
$50 off and 3 free domains with code: [color=#CC0000]DH3[/color] [url=http://dreamhost.com/r.cgi?296804/signup|DH3]Sign Up NOW[/url] or More Codes Here