The expression $this->doEnterNode($node); of type Twig_Node|Twig_NodeInterface adds the type Twig_NodeInterface to the return on line 24 which is incompatible with the return type declared by the interface Twig_NodeVisitorInterface::enterNode of type Twig_Node.
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}
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public function leaveNode(Twig_NodeInterface $node, Twig_Environment $env)
The return type of return $node; (Twig_NodeInterface) is incompatible with the return type declared by the interface Twig_NodeVisitorInterface::leaveNode of type Twig_Node|false.
If you return a value from a function or method, it should be a sub-type of the
type that is given by the parent type f.e. an interface, or abstract method.
This is more formally defined by the
Lizkov substitution principle,
and guarantees that classes that depend on the parent type can use any instance
of a child type interchangably. This principle also belongs to the
SOLID principles
for object oriented design.
Our function my_function expects a Post object, and outputs the author
of the post. The base class Post returns a simple string and outputting a
simple string will work just fine. However, the child class BlogPost which
is a sub-type of Post instead decided to return an object, and is
therefore violating the SOLID principles. If a BlogPost were passed to
my_function, PHP would not complain, but ultimately fail when executing the
strtoupper call in its body.