多选题的给分原则是多选错选不得分，部分选对得部分分

$q_answer = 'A,B,C'; //正确答案

$s_answer = 'A,D';  //学生答案
$q_answer = explode(',',$q_answer);
$s_answer = explode(',',$s_answer);

$count = count($q_answer);

$score = 6;

if(count(array_diff($s_answer,$q_answer)) > 0)
{
      $real_score = 0;
}
else
{
      $real_score = $score * count(array_intersect($q_answer,$s_answer)) / $count;
}
echo $real_score;

运行结果为：0