The best way to get the full PHP version string

Recently, to automate building, tagging, and pushing my geerlingguy/php-apache Docker Hub image (see this issue), I needed to find a way to reliably determine the PHP major.minor.release version string. You'd think this would be simple.

Well, using Docker, I would run the image and then try:

# php --version
PHP 7.3.0-1+0~20181206202713.23+stretch~1.gbp076afd (cli) (built: Dec  6 2018 20:27:14) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.0-dev, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.0-1+0~20181206202713.23+stretch~1.gbp076afd, Copyright (c) 1999-2018, by Zend Technologies

That's great; it outputs the version right at the start. But there are a few problems here:

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