小白问题，为什么对象都可以 echo ? [ 未指定版本 ]

ActiveField有__toString()方法~
/**

 * PHP magic method that returns the string representation of this object.
 * @return string the string representation of this object.
 */
public function __toString()
{
    // __toString cannot throw exception
    // use trigger_error to bypass this limitation
    try {
        return $this->render();
    } catch (\Exception $e) {
        ErrorHandler::convertExceptionToError($e);
        return '';
    }
}

public function render($content = null)
{
    if ($content === null) {
        if (!isset($this->parts['{input}'])) {
            $this->textInput();
        }
        if (!isset($this->parts['{label}'])) {
            $this->label();
        }
        if (!isset($this->parts['{error}'])) {
            $this->error();
        }
        if (!isset($this->parts['{hint}'])) {
            $this->hint(null);
        }
        $content = strtr($this->template, $this->parts);
    } elseif (!is_string($content)) {
        $content = call_user_func($content, $this);
    }

    return $this->begin() . "\n" . $content . "\n" . $this->end();
}