Как через php достать значение json

Для получения указанных данных нужно воспользоваться методом json_decode:

$str = '{"type":"message_new","object":{"id":20576562,"date":15159687644515,"out":0,"user_id":2675377610871,"read_state":0,"title":"","body":"","attachments":[{"type":"doc","doc":{"id":457793256,"owner_id":253776101,"title":"Название","size":9755,"ext":"gif","url":"https:","date":1515890058,"type":3,"preview":{"photo":{"sizes":[{"src":"https:","width":116,"height":100,"type":"m"},{"src":"https:","width":100,"height":75,"type":"s"},{"src":"https:","width":116,"height":119,"type":"o"}]},"video":{"src":"https:","width":116,"height":118,"file_size":34125}},"access_key":"59b34193bdd28012d2"}}]},"group_id":159709397,"secret":"aaQ472jhdkhkh"}';
// Можно как объект
$url = json_decode($str)->object->attachments[0]->doc->url; 
echo $url;
// Можно как массив
$url = json_decode($str, true)['object']['attachments'][0]['doc']['url']; 
echo $url;

$json = '{"a":1,"b":2,"c":3,"d":4,"e":5}';
var_dump(json_decode($json, true));

Результат

array(5) {
["a"] => int(1)
["b"] => int(2)
["c"] => int(3)
["d"] => int(4)
["e"] => int(5)
}