为什么mysql_query不能使用已定义的变量

Question

为什么当我在PHP文件中运行此查询时，我什么都没得到。任何建议。

当我在SQL phpMyAdmin中运行相同的代码时，它正常工作。我需要帮助，如何编辑我的代码才能运行。

$sql = "SET @prev_value = NULL;";
    $query = mysqli_query($connection,$sql) or trigger_error("Query Failed: " . mysqli_error($connection));
    $sql = "SET @rank_count = 0;";
    $query = mysqli_query($connection,$sql) or trigger_error("Query Failed: " . mysqli_error($connection));
    $sql = "SELECT *, 
    CASE
    WHEN @prev_value = totalPoints THEN @rank_count
    WHEN @prev_value := totalPoints THEN @rank_count := @rank_count + 1
    ELSE @rank_count := @rank_count + 1
    END AS userRank
    FROM resultsTestDB
    ORDER BY totalPoints;";
    $query = mysqli_query($connection,$sql) or trigger_error("Query Failed: " . mysqli_error($connection));

    $data = mysqli_fetch_assoc($query);

    for ($i=0; $i<sizeof($data); $i++){
        if ($data[$i]['memberID']==$memberID)
            $myRank = $data[$i]['userRank'];
    }

    return $myRank;