下面的代码可以完成这个任务，但是它取决于数据库和查询的内容。确保每个ID_KEY总是有3个值，并且结果被正确排序。这个想法是重复输出带有值的单元格，直到ID_KEY发生变化。许多小的变化都是可能的，但是最终它们都是按照这个原则工作的。

$result = $con->query($sql);

if ($result->num_rows > 0) {
    $res = "<table>";
        $res .= "<tr>";
            $res .= "<th>ID_KEY</th>";
            $res .= "<th>VALUE1</th>";
            $res .= "<th>VALUE2</th>";
            $res .= "<th>VALUE3</th>";
        $res .= "</tr>";
        $row = mysqli_fetch_assoc($result);
        while ($row) {
            $idKey = $row['ID_KEY'];
            $res .= "<tr>";
            $res .= "<td>" . $idKey . "</td>";
            $res .= "<td>" . $row['VALUE'] . "</td>";
            while (($row = mysqli_fetch_assoc($result)) &&
                   ($idKey == $row['ID_KEY'])) {
                $res .= "<td>" . $row['VALUE'] . "</td>";
            }
            $res .= "</tr>";
        }
    $res .= "</table>";

    return $res;
}

虽然不是最好的方法，但是你可以先对每个集合进行分组，然后循环输出分组后的数据，但是使用SQL对数据进行分组可能会更有效。

<?php

// Original Data
$originalData = [
    [
        'ID' => 1,
        'ID_KEY' => 1,
        'VALUE' => "Text1.1"
    ],
    [
        'ID' => 2,
        'ID_KEY' => 1,
        'VALUE' => "Text1.2"
    ],
    [
        'ID' => 3,
        'ID_KEY' => 1,
        'VALUE' => "Text1.3"
    ],
    [
        'ID' => 4,
        'ID_KEY' => 2,
        'VALUE' => "Text2.1"
    ],
    [
        'ID' => 5,
        'ID_KEY' => 2,
        'VALUE' => "Text2.2"
    ],
    [
        'ID' => 6,
        'ID_KEY' => 2,
        'VALUE' => "Text2.3"
    ],
    [
        'ID' => 7,
        'ID_KEY' => 3,
        'VALUE' => "Text3.1"
    ],
    [
        'ID' => 8,
        'ID_KEY' => 3,
        'VALUE' => "Text3.2"
    ],
        [
        'ID' => 9,
        'ID_KEY' => 3,
        'VALUE' => "Text3.3"
    ],
];

// Group the Data by ID_KEY
$groupedData = [];

foreach($originalData as $item) {
    $groupedData[$item['ID_KEY']][] = $item['VALUE'];
}

// HTML Output  
$output = "<table>";
    $output .= "<tr>";
        $output .= "<th>ID_KEY</th>";
        $output .= "<th>VALUE1</th>";
        $output .= "<th>VALUE2</th>";
        $output .= "<th>VALUE3</th>";
    $output .= "</tr>";

    // Output each row
    foreach($groupedData as $idKey => $groupData) {
         $output .= "<tr>";
            $output .= "<td>" . $idKey . "</td>";
            
            // Iterate over each value
            foreach($groupData as $value) {
                $output .= "<td>" . $value . "</td>";
            }
        $output .= "</tr>";
    }
$output .= "</table>";

// Output the HTML
'<pre>' . print_r($output, true) . '</pre>';