Why are there half a square brackets behind the many function parameters in the PHP manual? What's the point of these parentheses?

Why are there half a square brackets behind the many function parameters in the PHP manual? What's the point of these parentheses?
Always have this question, Google, Baidu has no answer, do not know if anyone can give a solution.
Thank

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Why are there half a square brackets behind the many function parameters in the PHP manual? What's the point of these parentheses?
Always have this question, Google, Baidu has no answer, do not know if anyone can give a solution.
Thank

array array_filter ( array $array [, callable $callback [, int $flag = 0 ]] )

You refer to the $callback $flag brackets in the two arguments, indicating that these are optional parameters

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