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Why do some array parameters have a comma after opening parentheses?

What is this syntax? I saw some documents, for example:

Jquery sizzle

Sizzle( String selector[, DOMNode context[, Array results]] )

Codeiginiter 3

set_userdata($data[, $value = NULL])

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2 answers

Basically, this means that all parameters inside [] are optional. You do not need to skip anything to make the function call work:

 foo($param [, $param2 = NULL, $param3 = 1])

$param1 and $param2 are optional, the $ param parameter is required.

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These brackets indicate that these parameters are optional in syntax. These examples are not lines of code, but syntax.

Valid lines of code:

 Sizzle(selector, context, results); Sizzle(selector, context); Sizzle(selector); set_userdata($data, $value); set_userdata($data);

But, if we look at @taxicala's answer, we will have a different situation.

 foo($param [, $param2 = NULL, $param3 = 1])

Valid lines of code

 foo($param, $param2, $param3) foo($param)

but not

 foo($param, $param2)
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