Adding a new executable to the PATH environment variable?
The cleanest way to achieve this would be to symlink the binary in the /usr/local/bin directory (which is included in PATH by default). You might need to create this directory if it doesn't exist yet. You can check to see if these directories are already in your PATH by opening Terminal.app and typing:

echo $PATH

This will generate a colon delimited listing of all directories in your PATH.

If the directories /usr/local or /usr/local/bin do not exist yet, execute the following:

sudo mkdir -p /usr/local/bin


Symlink the binary, replace (/Applications) with the path to phantomjs.app if it differs:

sudo ln -s /Applications/phantomjs.app/Contents/MacOS/phantomjs /usr/local/bin


Now you should have no problems executing phantomjs from the command-line.