Wygląda ok, w tym co wkleiłeś brakowało literki s file_get_contents
php -v
PHP 7.0.18-0ubuntu0.16.04.1 (cli) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
with Zend OPcache v7.0.18-0ubuntu0.16.04.1, Copyright (c) 1999-2017, by Zend Technologies
items[0];
var_dump($item->snippet->thumbnails);
object(stdClass)#6 (5) {
["default"]=>
object(stdClass)#5 (3) {
["url"]=>
string(46) "https://i.ytimg.com/vi/1pkiMqMfc2Y/default.jpg"
["width"]=>
int(120)
["height"]=>
int(90)
}
["medium"]=>
object(stdClass)#7 (3) {
["url"]=>
string(48) "https://i.ytimg.com/vi/1pkiMqMfc2Y/mqdefault.jpg"
["width"]=>
int(320)
["height"]=>
int(180)
}
["high"]=>
object(stdClass)#8 (3) {
["url"]=>
string(48) "https://i.ytimg.com/vi/1pkiMqMfc2Y/hqdefault.jpg"
["width"]=>
int(480)
["height"]=>
int(360)
}
["standard"]=>
object(stdClass)#9 (3) {
["url"]=>
string(48) "https://i.ytimg.com/vi/1pkiMqMfc2Y/sddefault.jpg"
["width"]=>
int(640)
["height"]=>
int(480)
}
["maxres"]=>
object(stdClass)#10 (3) {
["url"]=>
string(52) "https://i.ytimg.com/vi/1pkiMqMfc2Y/maxresdefault.jpg"
["width"]=>
int(1280)
["height"]=>
int(720)
}
}