在PHP中，可以使用array_reduce函数来按特定键分组数组。下面是一个示例代码：

 'John', 'age' => 20, 'score' => 85],
    ['name' => 'Alice', 'age' => 22, 'score' => 92],
    ['name' => 'Bob', 'age' => 20, 'score' => 78],
    ['name' => 'Jane', 'age' => 22, 'score' => 88],
    ['name' => 'Mark', 'age' => 21, 'score' => 90],
];

// 按年龄分组
$groupedByAge = array_reduce($students, function($result, $student) {
    $age = $student['age'];
    if (!isset($result[$age])) {
        $result[$age] = [];
    }
    $result[$age][] = $student;
    return $result;
}, []);

// 按分数分组
$groupedByScore = array_reduce($students, function($result, $student) {
    $score = $student['score'];
    if (!isset($result[$score])) {
        $result[$score] = [];
    }
    $result[$score][] = $student;
    return $result;
}, []);

// 输出结果
echo "按年龄分组：\n";
print_r($groupedByAge);
echo "\n按分数分组：\n";
print_r($groupedByScore);
?>

运行上述代码，将得到以下输出结果：

按年龄分组：
Array
(
    [20] => Array
        (
            [0] => Array
                (
                    [name] => John
                    [age] => 20
                    [score] => 85
                )

            [1] => Array
                (
                    [name] => Bob
                    [age] => 20
                    [score] => 78
                )

        )

    [22] => Array
        (
            [0] => Array
                (
                    [name] => Alice
                    [age] => 22
                    [score] => 92
                )

            [1] => Array
                (
                    [name] => Jane
                    [age] => 22
                    [score] => 88
                )

        )

    [21] => Array
        (
            [0] => Array
                (
                    [name] => Mark
                    [age] => 21
                    [score] => 90
                )

        )

)

按分数分组：
Array
(
    [85] => Array
        (
            [0] => Array
                (
                    [name] => John
                    [age] => 20
                    [score] => 85
                )

        )

    [92] => Array
        (
            [0] => Array
                (
                    [name] => Alice
                    [age] => 22
                    [score] => 92
                )

        )

    [78] => Array
        (
            [0] => Array
                (
                    [name] => Bob
                    [age] => 20
                    [score] => 78
                )

        )

    [88] => Array
        (
            [0] => Array
                (
                    [name] => Jane
                    [age] => 22
                    [score] => 88
                )

        )

    [90] => Array
        (
            [0] => Array
                (
                    [name] => Mark
                    [age] => 21
                    [score] => 90
                )

        )

)

以上代码通过array_reduce函数将原始数组按年龄和分数分别分组，并输出结果。