在使用准备好的语句进行查询的过程中，如何使用PHP填充数组

我将在此处发布整个php，但以下是我遇到的麻烦并着重关注的部分，即数组的实际填充量。

$mysqli = new mysqli('localhost', 'root', '', 'dota_site_test');
if(mysqli_connect_errno()) {
    echo "Connection Failed: " . mysqli_connect_errno();
    exit();
}
$player_id = 123;//change later

if($stmt = $mysqli -> prepare("SELECT Match_ID,Hero,Result,GameMode,MMR FROM matches WHERE Player_ID = ?")) {

    $stmt -> bind_param("i", $player_id);

    $stmt -> execute();

    $stmt -> bind_result($match_id, $hero, $result, $gamemode, $mmr);
    //adds the table setup only when data is fetched ok (above)
    echo "<table id='matches_table'>";
    echo "<thead>
                <tr>
                    <th>Hero</th>
                    <th>Result</th>
                    <th>Game Mode</th>
                    <th>MMR</th>
                    <th>Diff</th>
                </tr>
            </thead>";
    $i = 1;
    $mmr2 = 0;
    $grapharray = array();
    while($stmt -> fetch()){
    //While there is still data being fetched, make the table rows below
        $mmr1 = $mmr;       //first mmr calc value = mmr from fetch
        $diff = 0;          //difference defined as 0
        if ($mmr2 != 0){     //if the MMR2 has a value (from 2nd loop)
            $diff = $mmr1-$mmr2;   
        }
        $mmr2 = $mmr1;      //so that both values can be present in the loop

        echo "<tr><td>$hero</td>
                <td>$result</td>
                <td>$gamemode</td>
                <td data-id='$i'>$mmr</td>
                <td>$diff</td></tr>";   
        $i++;
        $grapharray[$stmt['$hero']] = $stmt['$mmr'];

    }

    echo "</table>";

    $stmt -> close();

}
$mysqli->close();

?>

这是我关注的部分，我试图使用while循环，该循环使用数据填充HTML表，还使用该循环将数据单元格（MMR和Hero名称）放入数组中，以供以后创建图形时使用。

这是更集中的部分：

while($stmt -> fetch()){
    //While there is still data being fetched, make the table rows below
        $mmr1 = $mmr;       //first mmr calc value = mmr from fetch
        $diff = 0;          //difference defined as 0
        if ($mmr2 != 0){     //if the MMR2 has a value (from 2nd loop)
            $diff = $mmr1-$mmr2;   
        }
        $mmr2 = $mmr1;      //so that both values can be present in the loop

        echo "<tr><td>$hero</td>
                <td>$result</td>
                <td>$gamemode</td>
                <td data-id='$i'>$mmr</td>
                <td>$diff</td></tr>";   
        $i++;
        $grapharray[$stmt['$hero']] = $stmt['$mmr'];

    }

我不确定如何用此循环制作一个数组，因为出现错误：

Fatal error: Cannot use object of type mysqli_stmt as array in C:\xampp\htdocs\Dota2HomePage\d2index.php on line 79

我想我做错了什么！