binpath not available a variable

[bmguillory+macports@…]

I tried to manipulate the PATH environment variable using this code:

test.env            PATH=${worksrcpath}:${binpath}

But when I try to run my test I get this output:

notbrad$ sudo port -vvv test safekeep
Error: Unable to open port: can't read "binpath": no such variable

The guide indicates that binpath should be a read only variable in section "5.2. Global Variables"