Pseudo-types and variables used in this documentation

mixed

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.

number

number indicates that a parameter can be either integer or float.

callback

callback pseudo-types was used in this documentation before callable type hint was introduced by PHP 5.4. It means exactly the same.

array|object

array|object indicates that a parameter can be either array or object.

void

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.