Another option for merging arrays is union.

$attributes = ['title' => 'My Blog Post', 'body' => 'Lorem ipsum.'];

$additional = ['category_id' => 1];

Post::create($attributes + $additional);

or

$attributes = ['title' => 'My Blog Post', 'body' => 'Lorem ipsum.'];

Post::create($attributes + [
    'category_id' => 1
]);

Docs: https://www.php.net/manual/en/language.operators.array.php

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.

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