类方法怪异（PHP）

I encountered a class which has the following method:

class Period
{
/** @var \DateTime */
public $startDate;

/** @var \DateTime */
public $endDate;

public static function create(DateTime $startDate, $endDate): Period
{
    return new static($startDate, $endDate);
}
}

My question is about the part after the create(...) - the ": Period". I have never seen this before and can't find any documentation about it (mainly because I have no clue what to look for). Can anyone shed some light on this?

This is new in PHP version 7 and above. It's called return type declarations. In the example you posted it requires the method create to return an object of Period.

An easier example would be:

function giveMeAnInt() : int {
    return 1;
}

Above code is valid because it returns an integer.

function giveMeAnInt() : int {
    return "Hello";
}

This is not valid and will return a fatal error "FATAL ERROR Uncaught TypeError: Return value of giveMeAnInt() must be of the type integer, string returned"

looks like this code belongs to time range api for php (class) , for more informations http://period.thephpleague.com/

Example : to create a new instance representing a given week :

<?php

public static Period::createFromWeek(int $year, int $week): Period
...

The $year parameter must be a valid year;
The $week parameter must be a valid week (between 1 and 53);

more examples here : http://period.thephpleague.com/examples/