如何在.gitlab-ci.yml变量中使用条件？

我想知道，是否可以从if-else条件语句设置自定义Gitlab CI变量。

在我的.gitlab-ci.yml文件中，我有以下内容：

variables:
    PROJECT_VERSION: (if [ "${CI_COMMIT_TAG}" == "" ]; then "${CI_COMMIT_REF_NAME}-${CI_PIPELINE_ID}"; else ${CI_COMMIT_TAG}; fi);

Trying to set project version:
    image: php:7.1-cli
    stage: test
    script:
        # this echoes correct string (eg. "master-2794")
        - (if [ "${CI_COMMIT_TAG}" == "" ]; then echo "${CI_COMMIT_REF_NAME}-${CI_PIPELINE_ID}"; else echo ${CI_COMMIT_TAG}; fi);
        # this echoes something like "(if [ "" == "" ]; then "master-2794"; else ; fi);"
        - echo $PROJECT_VERSION

能做到吗？如果是这样，我错过了什么？谢谢