二维数组排序，比如一个学生成绩数组：
$arr[] = array('name'=>'a','score'=>9);
$arr[] = array('name'=>'c','score'=>4);
$arr[] = array('name'=>'d','score'=>1);
$arr[] = array('name'=>'b','score'=>3);
$arr[] = array('name'=>'f','score'=>2);
$arr[] = array('name'=>'e','score'=>7);
$arr[] = array('name'=>'h','score'=>6);
$arr[] = array('name'=>'g','score'=>8);
$arr[] = array('name'=>'a'
