PHP 8 implode() 类型错误：从警告到致命错误及解决方案

$value = 'single_string';
var_dump(implode(',', $value));
// Output in PHP 7.x:
// Warning: implode(): Invalid arguments passed in ... on line X
// NULL

$value = 'single_string';
var_dump(implode(',', $value));
// Output in PHP 8:
// Fatal error: Uncaught TypeError: implode(): Argument #2 ($array) must be of type ?array, string given in ... on line X

'characteristics' => implode(',', $characteristics[$key]),

foreach($activity_selected as $key => $val) 
{
    // 确保 $characteristics[$key] 存在且是数组，否则使用空数组
    $currentCharacteristics = $characteristics[$key] ?? []; 

    $dataSet[] = array (
            'batch_id' => $batch_id,
            'activity_id' => $activity_selected[$key],
            'characteristics' => implode(',', is_array($currentCharacteristics) ? $currentCharacteristics : []),
            'user_id' => $user_id,
    );  
}

<?php

// 模拟 $this->input->post('characteristics') 可能返回的数据
// 情况一：期望的数组（例如，多选框提交）
$characteristics_case_1 = [
    0 => ['red', 'green'],
    1 => ['small'],
    2 => ['large', 'blue', 'fast']
];

// 情况二：包含非数组值（例如，单选文本框或数据处理不当）
$characteristics_case_2 = [
    0 => 'red_string_value', // 这是一个字符串，而不是数组
    1 => ['small'],
    2 => ['large', 'blue', 'fast']
];

// 模拟循环中的 $activity_selected 键
$activity_selected_mock = [0, 1, 2];

echo "--- 处理 characteristics_case_1 (期望情况) ---\n";
foreach ($activity_selected_mock as $key => $val) {
    $currentCharacteristicValue = $characteristics_case_1[$key] ?? [];
    $implodedValue = implode(',', is_array($currentCharacteristicValue) ? $currentCharacteristicValue : []);
    echo "Activity Key $key, Characteristics: " . $implodedValue . "\n";
}
/* 预期输出：
--- 处理 characteristics_case_1 (期望情况) ---
Activity Key 0, Characteristics: red,green
Activity Key 1, Characteristics: small
Activity Key 2, Characteristics: large,blue,fast
*/

echo "\n--- 处理 characteristics_case_2 (包含非数组值) ---\n";
foreach ($activity_selected_mock as $key => $val) {
    // 原始代码在 PHP 8 中会在这里抛出 TypeError
    // try {
    //     $implodedValueOriginal = implode(',', $characteristics_case_2[$key]);
    //     echo "Activity Key $key, Original Imploded: " . $implodedValueOriginal . "\n";
    // } catch (TypeError $e) {
    //     echo "Activity Key $key, Original Error: " . $e->getMessage() . "\n";
    // }

    // 修复后的代码
    $currentCharacteristicValue = $characteristics_case_2[$key] ?? [];
    $implodedValueFixed = implode(',', is_array($currentCharacteristicValue) ? $currentCharacteristicValue : []);
    echo "Activity Key $key, Fixed Imploded: " . $implodedValueFixed . "\n";
}
/* 预期输出：
--- 处理 characteristics_case_2 (包含非数组值) ---
Activity Key 0, Fixed Imploded: 
Activity Key 1, Fixed Imploded: small
Activity Key 2, Fixed Imploded: large,blue,fast
*/