eZ\Publish\Core\REST\Client\Values\Content\Content::getFieldValue PHP Method

    public function getFieldValue($fieldDefIdentifier, $languageCode = null)
    {
        if (null === $languageCode) {
            $languageCode = $this->versionInfo->contentInfo->mainLanguageCode;
        }
        if (isset($this->fields[$fieldDefIdentifier][$languageCode])) {
            return $this->fields[$fieldDefIdentifier][$languageCode]->value;
        }
        return null;
    }