<?php
$array = [
    "1990" => "США",
    "1991" => "Мексика",
    "1996" => "Нью-Йорк",
    "1997" => "Прага",
    "1999" => "Ялта",
    "2000" => "Сочи",
    "2001" => "Киев",
    "2002" => "Москва",
    "2003" => "Пекин",
    "2004" => "США",
];

$current = "1990";