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mixed indicates that a parameter may accept multiple (but not necessarily all) types.
gettype() for example will accept all PHP types, while str_replace() will accept strings and arrays.
callback pseudo-types was used in this documentation before callable type hint was introduced by PHP 5.4. It means exactly the same.
void as a return type means that the return value is useless. void in a parameter list means that the function doesn't accept any parameters.
$...
in function prototypes means
and so on. This variable name is used when a function can
take an endless number of arguments.