Is your shell command actually outputting anything? If you're just executing a program which does not print anything to stdout then you won't have anything in the stream to read. What happens if you try a 'ls -l' or some other command which definitely outputs results?
Maybe try phpseclib: a pure php ssh implementation? Unlike the PECL ssh2 extension, phpseclib's author actively provides support, as can be seen by looking at the support forum. Also, frankly, it offers more debugging capabilities than the PECL ssh2 extension does, with such things as $ssh->getLog() and $ssh->getLastError().