<?php

$data = array(
    array(  
        "continent" => "Europa",
        "country" => "France",
        "capital" => "Paris",
        ),
   array(
       "continent" => "Europa",
       "country" => "Spain",
       "capital" => "Madrid",
       ),
   array(
       "continent" => "Asia",
       "country" => "Russia",
       "capital" => "Moscow",
       )
);


$group_by = 'continent';
$attributes = array( 'country', 'capital' );

$output = array();
foreach( $data as $country ) {

    if( isset( $country[$group_by] ) ) {

        $one = array();
        foreach( $attributes as $attribute ) {
            if( isset( $country[$attribute] ) ) {
                $one[$attribute] = $country[$attribute];
            }
        }
        $output[ $country[$group_by] ][] = $one;
     }
}


print_r($output);


?>