Pseudo-types and variables used in this documentation
mixed indicates that a parameter may accept multiple (but not necessarily all) types.
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.