AbstractPlugin 

class RandomContent extends AbstractPlugin
{
    public function main(string $content, array $conf) : string
    {
        $this->conf = $conf;

        $this->pi_initPIflexForm(); // Init FlexForm configuration for plugin
        if ($this->pi_getFFvalue($this->cObj->data['pi_flexform'], 'which_pages', 'sDEF')) {
            $this->conf['pages'] = $this->pi_getFFvalue($this->cObj->data['pi_flexform'], 'which_pages', 'sDEF');
        }
        // ...
    }
}

class RandomContent
{
    /**
     * The back-reference to the mother cObj object set at call time
     */
    public $cObj;

    /**
     * This setter is called when the plugin is called from UserContentObject (USER)
     * via ContentObjectRenderer->callUserFunction().
     *
     * @param ContentObjectRenderer $cObj
     */
    public function setContentObjectRenderer(ContentObjectRenderer $cObj): void
    {
        $this->cObj = $cObj;
    }

    public function main(string $content, array $conf) : string
    {
         $this->conf = $conf;

        $this->pi_initPIflexForm(); // Init FlexForm configuration for plugin
        if ($this->pi_getFFvalue($this->cObj->data['pi_flexform'], 'which_pages', 'sDEF')) {
            $this->conf['pages'] = $this->pi_getFFvalue($this->cObj->data['pi_flexform'], 'which_pages', 'sDEF');
        }
        // ...
    }

    /**
     * Converts $this->cObj->data['pi_flexform'] from XML string to flexForm array.
     *
     * @param string $field Field name to convert
     */
    public function pi_initPIflexForm($field = 'pi_flexform')
    {
        // ...
    }

    public function pi_getFFvalue($T3FlexForm_array, $fieldName, $sheet = 'sDEF', $lang = 'lDEF', $value = 'vDEF')
    {
        // ...
    }

}