Pseudo-types and variables used in this documentation
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
number indicates that a parameter can be either
integer or float.
callback pseudo-types was used in this documentation before
callable type hint was introduced by PHP 5.4. It means exactly
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.