PHP飞船操作符（）返回值深度解析：仅限-1, 0, 1

<?php

// 1. 整数比较
echo "5 <=> 10: " . (5 <=> 10) . "\n";   // 输出: -1 (5 小于 10)
echo "10 <=> 10: " . (10 <=> 10) . "\n"; // 输出: 0 (10 等于 10)
echo "15 <=> 10: " . (15 <=> 10) . "\n"; // 输出: 1 (15 大于 10)

echo "\n";

// 2. 浮点数比较
echo "3.14 <=> 2.71: " . (3.14 <=> 2.71) . "\n"; // 输出: 1 (3.14 大于 2.71)
echo "1.0 <=> 1.0: " . (1.0 <=> 1.0) . "\n";     // 输出: 0 (1.0 等于 1.0)
echo "0.5 <=> 1.5: " . (0.5 <=> 1.5) . "\n";     // 输出: -1 (0.5 小于 1.5)

echo "\n";

// 3. 字符串比较（按字典顺序）
echo "'apple' <=> 'banana': " . ('apple' <=> 'banana') . "\n"; // 输出: -1 ('apple' 小于 'banana')
echo "'cat' <=> 'cat': " . ('cat' <=> 'cat') . "\n";         // 输出: 0 ('cat' 等于 'cat')
echo "'dog' <=> 'bird': " . ('dog' <=> 'bird') . "\n";       // 输出: 1 ('dog' 大于 'bird')

echo "\n";

// 4. 与 null 比较 (null 被视为小于任何非 null 值)
echo "null <=> 0: " . (null <=> 0) . "\n";         // 输出: -1
echo "0 <=> null: " . (0 <=> null) . "\n";         // 输出: 1
echo "null <=> null: " . (null <=> null) . "\n";   // 输出: 0

echo "\n";

// 5. 在用户定义排序函数中的应用 (usort)
$numbers = [3, 1, 4, 1, 5, 9, 2, 6];
usort($numbers, function($a, $b) {
    return $a <=> $b; // 升序排序
});
echo "排序后的数组（升序）: " . implode(", ", $numbers) . "\n"; // 输出: 1, 1, 2, 3, 4, 5, 6, 9

usort($numbers, function($a, $b) {
    return $b <=> $a; // 降序排序
});
echo "排序后的数组（降序）: " . implode(", ", $numbers) . "\n"; // 输出: 9, 6, 5, 4, 3, 2, 1, 1

?>

<?php
echo "'5' <=> 10: " . ('5' <=> 10) . "\n"; // 输出: -1 (字符串 '5' 被转换为整数 5，5 小于 10)
echo "'10' <=> 10: " . ('10' <=> 10) . "\n"; // 输出: 0 (字符串 '10' 被转换为整数 10，10 等于 10)
echo "'15' <=> 10: " . ('15' <=> 10) . "\n"; // 输出: 1 (字符串 '15' 被转换为整数 15，15 大于 10)
?>