随机生成20以内加减法运算题目

<?php

function generateMathProblem()
{
    // 随机选择加法或减法
    $operation = rand(0, 1) ? '+' : '-';

    // 生成两个0到20之间的随机数
    $num1 = rand(0, 20);
    $num2 = rand(0, 20);

    // 计算结果，注意处理减法可能导致负数的情况
    if ($operation == '-') {
        // 确保减法结果不为负数
        if ($num1 < $num2) {
            // 交换两个数以确保结果为正数
            list($num1, $num2) = [$num2, $num1];
        }
    }
    // 计算结果,如果结果大于20，改为减法
    if ($operation == '+') {
        // 确保减法结果不为负数
        if (bcadd($num1,$num2) > 20) {
            $operation = '-';
            if($num2 >= $num1){
                // 交换两个数以确保结果为正数
                list($num1, $num2) = [$num2, $num1];
            }
        }
    }

    // 构建题目字符串
    $problem = "$num1 $operation $num2 = ";

    // 计算结果
    $answer = eval("return $num1 $operation $num2;");

    return [
        'problem' => $problem,
        'answer' => $answer
    ];
}

if ((int)$_GET['timu_num']) {
    $timu_num = (int)$_GET['timu_num'];
} else {
    $timu_num = 100;
}

// 生成多少道题目
$problems = [];
for ($i = 0; $i < $timu_num; $i++) {
    $problems[] = generateMathProblem();
}

// 输出题目和答案
echo "<table>";
foreach ($problems as $index => $problem) {
    $num = $index+1;
    if($num%5 == 1) echo "<tr>";
    echo  "<td style='width: 120px'>" . $problem['problem'] . "</td>";
    if($num%5 == 0) echo "</tr>";
}

echo "<tr><td>答案：</td></tr>";
foreach ($problems as $index => $problem) {
    $num = $index+1;
    if($num%5 == 1) echo "<tr>";
    echo  "<td style='width: 120px'>" . "题目 " . $num . ": " . $problem['answer'] . "</td>";
    if($num%5 == 0) echo "</tr>";
}

echo "</<table>";