给定一个学生成绩数组：
```
$students = [
    ['name' => 'Alice', 'score' => 88],
    ['name' => 'Bob', 'score' => 72],
    ['name' => 'Charlie', 'score' => 95],
    ['name' => 'David', 'score' => 60],
];
```
要求：
```
计算平均分
找出最高分学生
找出及格学生
按成绩从高到低排序
给每个学生添加等级：
	90 以上：A
	80–89：B
	70–79：C
	60–69：D
	60 以下：F
```
练习点：
```
array_map
array_filter
array_reduce
usort
过程式拆函数
```

代码：
```
<?php

$students = [
    ['name' => 'Alice', 'score' => 88],
    ['name' => 'Bob', 'score' => 72],
    ['name' => 'Charlie', 'score' => 95],
    ['name' => 'David', 'score' => 60],
];

/**
 * 根据分数返回等级
 */
function getGrade(int $score): string
{
    return match (true) {
        $score >= 90 => 'A',
        $score >= 80 => 'B',
        $score >= 70 => 'C',
        $score >= 60 => 'D',
        default => 'F',
    };
}

/**
 * 计算平均分
 */
function averageScore(array $students): float
{
    $total = array_reduce(
        $students,
        fn(int $sum, array $student) => $sum + $student['score'],
        0
    );

return $total / count($students);
}

/**
 * 找出最高分学生
 */
function topStudent(array $students): array
{
    return array_reduce($students, function (?array $top, array $student) {
        if ($top === null || $student['score'] > $top['score']) {
            return $student;
        }

return $top;
    });
}

// 找出及格学生
$passedStudents = array_filter(
    $students,
    fn(array $student) => $student['score'] >= 60
);

// 按分数从高到低排序
$sortedStudents = $students;

usort($sortedStudents, fn(array $a, array $b) => $b['score'] <=> $a['score']);

// 给每个学生添加等级
$studentsWithGrade = array_map(function (array $student) {
    $student['grade'] = getGrade($student['score']);
    return $student;
}, $students);

echo "平均分：" . averageScore($students) . PHP_EOL;
echo "最高分学生：" . topStudent($students)['name'] . PHP_EOL;

print_r($passedStudents);
print_r($sortedStudents);
print_r($studentsWithGrade);
```

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