I have defined a `phpenv` custom tool which has a `PHP_VERSION` parameter which allows the requested PHP version to be specified. The installation script for this custom tool is roughly as follows:

```
export PHPENV_ROOT=$(pwd)/phpenv
export PATH="$PATH:$(pwd)/phpenv/bin"

if test -d $PHPENV_ROOT; then
$PHPENV_ROOT/bin/phpenv update
else
(curl -L http://git.io/phpenv-installer | bash)
fi

phpenv install $PHP_VERSION
phpenv global $PHP_VERSION
```

It seems, however, that the `$PHP_VERSION` environment variable is not set. Am I correct in thinking that the version parameter is exposed through an environment variable?