From PHP manual:
array array_change_key_case ( array $array [, int $case = CASE_LOWER ] )
What does comma signify in a function definition?
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Comma is a separator between parameters in the parameter list.
Comma is part of the parameter lists. Optional parameters are usually specified inside square brackets in documentation like so:
[, int $case = CASE_LOWER]. Comma is added to specify that if you were to use the second optional parameter, you must have a comma separator as well.
The fact that comma is inside square brackets, means it's only required when you have more than one parameter.
Another example with more optional parameters:
array array_change_key_case ( array $array [, int $case = CASE_LOWER [, string $myValue = '' ]] )