Is it possible that Bastian's journey to Fantastica was purely metaphysical?

Question

The first half of The Neverending Story is about Bastian reading a fantasy story about Atreyu's adventures in Fantastica. The second half is about Bastian's own adventures in Fantastica, a world which he basically recreates from nothing, becoming a completely different person himself in the process, before returning to his own world to find that he's still the same person as originally and only a single night has passed.

My question is: is it possible that Bastian never actually went to Fantastica? Is this a valid and consistent reading of the story, or does it not make sense with everything that happened?

• Bastian's adventures leave a permanent effect on him, making him more brave and confident. But the same effect could have come from a vivid hallucination in which he had all the experiences detailed in the second half of the book.
• The conversation at the very end between Bastian and Mr Coreander hints at a more metaphorical interpretation of Fantastica and the whole story, but I can't quite solidify this idea.

Years after reading The Neverending Story as a child, I came to the conclusion that there are many different ways to interpret the story, and I think this may be one of them: that Bastian's whole experience was a dream or hallucination after reading a story he loved, which changed his personality literally overnight. But I'm not certain if it's a totally consistent model of the narrative.

Answer 1

During Bastian's return from Fantastica, the following sequence of events takes place (starting on page 388 in my Dutton 1997 edition):

Quickly Bastian cupped his hands, gathered as much of the Water of Life as he could hold, ran to the gate, and flung himself into the empty darkness beyond.
...
[He] put on his his shoes and coat, and saw to his surprise that they were wet as they had been the day when it had rained so hard.
...
Even if he had spilled the Water of Life, he wanted to go home to his father

I had always read this as Bastian somehow bringing the Water across the barrier between the two worlds, and thus as the one solid counter to the idea that it all takes place in Bastian's head. However, as pointed out in the comments, Bastian is unaware that his entire journey took place overnight, and thus that his garments are indeed still wet from his previous night's rains.

With that said, given that this is a work of reconstructivist art, I don't think the dual interpretations are either unintentional or incompatible. Ende is telling us that this is simultaneously both Real AND in all in Bastian's head, those are not different things within the larger framework Ende is creating.