您需要第一个foreach来浏览$eleves。对于每个学生，您需要第二个foreach来添加所有分数并计算平均值。

function evalMoyenne ($grades) {
    $gradeSum = 0;
    $coeffCount = 0;

    foreach ( $grades as $subject => $grade ) {
        $gradeSum += $grade['note'] * $grade['cof'];
        $coeffCount += $grade['cof'];
    }

    return $gradeSum / $coeffCount;
}

foreach ( $eleves as $matricule => $row ) {
    $moyenne = evalMoyenne( $row['module'] );
    echo "<td>$matricule</td>"
    . '<td>' . $row[0] . '</td>'
    . '<td>' . $moyenne . '</td>'
}

通常，如果数组中有一个数组，则需要在foreach中有一个foreach。作为一种替代方法，如果有固定的主题计数，也可以硬编码含义计算：

$gradeSum = $row['module']['Math']['note'] * $row['module']['Math']['cof']
    + $row['module']['Physique']['note'] * $row['module']['Physique']['cof']
    + $row['module']['Langue']['note'] * $row['module']['Langue']['cof']
$cofSum = $row['module']['Math']['cof'] + $row['module']['Physique']['cof'] + $row['module']['Langue']['cof'];
$moyenne = $gradeSum / $cofSum;