How to remove elements from an array by groups?

Question

PHP

There is a table of cities, they indicate countries in the form of id of these countries from the countries table. How can I display cities by country from an array. It should be like this:
Russia: Moscow, St. Petersburg....
Belarus: Minsk, Mogilev....
Kazakhstan: Astana, Temirtau....
It is desirable that the output be automatically recognized by the country id. In order not to manually check each country, in case new countries are added.

Answer 1

Make a selection from the database:

SELECT 
  country.name, cities.name
FROM 
  country 
LEFT JOIN 
  cities
ON 
  country.id=cities.country_id

Answer 2

$query = "SELECT * from contry";
$res = mysql_query( $query );

     while ( $item = mysql_fetch_array( $res ) )

    { 
if ($item['country.name']=="Россия") {$rus.= $item['city.name'].' '; };
if ($item['country.name']=="Беларусь") {$bel.= $item['city.name'].' '; };       		
if ($item['country.name']=="Казахстан") {$kz.= $item['city.name'].' '; };
}

echo 'Россия: '.$rus;
echo 'Беларусь: '.$bel;
echo 'Казахстан :'.$kz;