How to create such a nested array?

Question

PHP

At the input there is xml data that I want to process and output to an array. Example of initial data:

<Cities City_ID="1196910018" City_owner_ID="0" CityName="52127 городок" RegionName="Московская обл." Region_ID="40" Region_Owner_ID="0" Country_ID="209" Country_Owner_ID="0"/>
<Cities City_ID="1940" City_owner_ID="0" CityName="Якшур-Бодья" RegionName="Удмуртская респ." Region_ID="84" Region_Owner_ID="0" Country_ID="209" Country_Owner_ID="0"/>
<Cities City_ID="1196906999" City_owner_ID="0" CityName="4-й Участок" RegionName="Московская обл." Region_ID="40" Region_Owner_ID="0" Country_ID="209" Country_Owner_ID="0"/>

At the output, you need an array:
[0] => Moscow region ('52127 town', '4th Plot');
[1] => Udmurt Republic ('Yakshur-Bodya');
Those. array where key = RegionName and value is an array of CityName for that RegionName