编译错误：无法对表达式的结果使用 isset()

我在从 SF2.0.x 迁移到 SF2.7 的应用程序中遇到此错误：

[1] Symfony\Component\Debug\Exception\FatalErrorException: Compile Error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
    at n/a
        in /var/www/html/reptooln_admin/app/cache/dev/twig/68/7f/63589dd3687cb849dd68e6b6c10aa99eda1d82f95a5f3ac52a864d200499.php line 39

我不知道出了什么问题或如何解决这个问题，所以我需要一些建议。这是缓存文件中报告堆栈跟踪的行：

    if ((((empty((isset($context["form_action"]) ? $context["form_action"] : $this->getContext($context, "form_action"))) == true) || (isnull((isset($context["form_action"]) ? $context["form_action"] : $this->getContext($context, "form_action"))) == true)) || (isset((isset($context["form_action"]) ? $context["form_action"] : $this->getContext($context, "form_action"))) == false))) {
                echo " ";
                $context["form_action"] = "";
                echo " ";

我有这个 TwigExtension：

class PDOneTwigExtension extends \Twig_Extension
{
    public function getFilters()
    {
        return array(
            'var_dump'   => new \Twig_Filter_Function('var_dump'),
            'empty' => new \Twig_Filter_Function('empty', array($this, 'is_empty')),
            'isset' => new \Twig_Filter_Function('isset', array($this, 'is_set')),
            'isnull' => new \Twig_Filter_Function('isnull', array($this, 'is_null')),
            'ucfirst' => new \Twig_Filter_Function('ucfirst', array($this, 'uc_first')),
            'ucwords' => new \Twig_Filter_Function('ucwords', array($this, 'uc_words')),
            'count' => new \Twig_Filter_Function('count', array($this, 'co_unt')),
            'sizeof' => new \Twig_Filter_Function('sizeof', array($this, 'size_of')),
            'concat' => new \Twig_Filter_Function('concat', array($this, 'concat')),
            'in_array' => new \Twig_Filter_Function('in_array', array($this, 'inarray')),
            'array' => new \Twig_Filter_Function('array', array($this, 'array_')),
            'add_to_array' => new \Twig_Filter_Function('add_to_array', array($this, 'add_to_array')),
            'replace' => new \Twig_Filter_Function('replace', array($this, 'replace')),
            'htmlentitydecode' => new \Twig_Filter_Function('htmlentitydecode', array($this, 'htmlentitydecode'))
        );
    }

    public function is_empty($sentence)
    {
        return empty($sentence) ? true : false;
    }

    // rest of methods goes here

    public function getName()
    {
        return 'pdone_twig_extension';
    }
}

我使用的模板如下：

{% if form_action|empty == true or form_action|isnull == true or form_action|isset == false %} {% set form_action = '' %} {% endif %}

这里的问题可能出在哪里？有什么建议吗？

From 文档 http://php.net/manual/en/function.isset.php:

isset() 仅适用于变量，因为传递任何其他内容都会导致解析错误。

您没有直接将变量传递给isset()。所以你需要先计算值，将其分配给一个变量，然后将其传递给isset().

例如，你现在正在做的事情是这样的：

if(isset($something === false)) { } // throws a parse error, because $something === false is not a variable

你需要做的是：

$something = false;
if(isset($something)) { ... }