Not familiar with the debian builds, I might go and install one in a minute, however
suggests to me that php gets installed...
You might like a "apt-get install php5-cli"
or, to my tiny mind, you could be looking for php5 binary
so try "php5 -q install.php"
Not altogether sure if something like "ln -s /usr/bin/php5 /usr/bin/php would solve any requirements