Group values of a flat array by the mapped values in another flat array

Which is the most effective way to group array elements respect to the values of another array in PHP?

Example:

names = [antonio, luigi, marco, stefano, gennaro, pino, lorenzo];
surnames = [bianchi, rossi, rossi, brambilla, rossi, rossi, brambilla];

expected result:

bianchi:
antonio

rossi: 
luigi, marco, gennaro, pino

brambilla:
stefano, lorenzo

Try this

foreach ($surnames as $key => $value) { 
  if (isset($result[$value])) { 
    if (!is_array($result[$value])) $result[$value] = (array) $result[$value]; 
    array_push($result[$value], $names[$key]);
  } 
  else 
  $result[$value]= $names[$key]; 
}
print_r($result);

Output

Array (
    [bianchi] => antonio
    [rossi] => Array ( [0] => luigi, [1] => marco, [2] => gennaro, [3] => pino )
    [brambilla] => Array ( [0] => stefano, [1] => lorenzo )    
)

Otherwise

foreach ($surnames as $key => $value) { $result[$value][] = $names[$key]; }

Output

Array (
    [bianchi] => Array ( [0] => antonio )
    [rossi] => Array ( [0] => luigi, [1] => marco, [2] => gennaro, [3] => pino )
    [brambilla] => Array ( [0] => stefano, [1] => lorenzo )    
)