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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Not valid. The task clearly says: "You get an array of arrays". You cannot get a scalar value there.
Try this test:
$this->assertSame(getLengthOfMissingArray([1, [3,1,2]]), 0);
Your function will return 2. But why? The function should return 0, because 1st item of the array is scalar value, not array.
You don't check that the array contains only arrays.