$domains = array(
  'my.domain' => '144.1.2.5',
  'her.domain' => '28.83.21.99',
  'new.domain' => '36.4.5.2.6',
  'old.domain' => '92.76.210.88',
);

//выведет 36.4.5.2.6
echo $domains['new.domain'] . '<br />';