Specify the path where you have phing installed.
See Setting-Up Phing
And you can use environment variables such like ''$WORKSPACE'' or ''$HOME''.
If you have installed phing into $WORKSPACE/vendor/bin/phing by composer,
specify "$WORKSPACE/vendor" or "$WORKSPACE/vendor/bin".
If you have installed it into /usr/bin/phing by PEAR,
specify "/usr" or "usr/bin".