<?php
$a = array(
      0=>array("uid"=>1,"score"=>22),
      1=>array("uid"=>2,"score"=>33),
      2=>array("uid"=>3,"score"=>55),
      3=>array("uid"=>3,"score"=>55),
      7=>array("uid"=>1,"score"=>39),
      8=>array("uid"=>2,"score"=>33),
     );
foreach($a as $key=> $values)
{

  $sum[$values['uid']] += $values['score'];    
}
print_r($sum);
?>