Neos\Flow\Configuration\ConfigurationManager::replaceVariablesInValue PHP Method

    protected function replaceVariablesInValue($value)
    {
        $matches = [];
        $replacements = [];
        preg_match_all('/
            (?P<fullMatch>%             # an expression is indicated by %
            (?P<expression>
            (?:(?:\\\\?[\\d\\w_\\\\]+\\:\\:)  # either a class name followed by ::
            |                           # or
            (?:(?P<prefix>[a-z]+)\\:)    # a prefix followed by : (like "env:")
            )?
            (?P<name>[A-Z_0-9]+))       # the actual variable name in all upper
            %)                          # concluded by %
        /mx', $value, $matches, PREG_SET_ORDER);
        foreach ($matches as $matchGroup) {
            if (isset($matchGroup['prefix']) && $matchGroup['prefix'] === 'env') {
                $replacements[$matchGroup['fullMatch']] = getenv($matchGroup['name']);
            }
            if (defined($matchGroup['expression'])) {
                $replacements[$matchGroup['fullMatch']] = constant($matchGroup['expression']);
            }
        }
        $replacementCount = count($replacements);
        if ($replacementCount === 0) {
            return $value;
        }
        if ($replacementCount === 1 && array_keys($replacements)[0] === $value) {
            // the replacement spans the complete directive, assign directly to keep CONSTANT/ENV variable type
            return reset($replacements);
        }
        return str_replace(array_keys($replacements), $replacements, $value);
    }