Pseudo-types are keywords used in the PHP documentation to specify the types or values an argument can have. Please note that they are not primitives of the PHP language. So you cannot use pseudo-types as typehints in your own custom functions.
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. As of PHP 7.1 void is accepted as a function return type hint.
$... in function prototypes means
and so on. This variable name is used when a function can
take an endless number of arguments.