AJAX并发文件上传：PHP $_FILES的独立性与处理机制解析

<form id="myform" enctype="multipart/form-data">
    <input id="files" name="files" type="file" class="form-control" multiple>
</form>

// 假设用户选择了多个文件
var selectedFiles = $('#files')[0].files;

for (var i = 0; i < selectedFiles.length; i++) {
    var file = selectedFiles[i];
    // 调用上传函数，为每个文件发起独立的AJAX请求
    uploadFile(file, recipient, $row); 
}

function uploadFile(file, recipient, $row) {
    var data = new FormData();
    data.append("file", file); // 将单个文件添加到FormData对象

    $.ajax({
        url: 'upload.php',
        type: 'POST',
        data: data,
        cache: false,
        contentType: false, // 必须设置为false，FormData会自动设置正确的Content-Type
        processData: false, // 必须设置为false，jQuery不处理FormData
        xhr: function() {
            var myxhr = $.ajaxSettings.xhr();
            if (myxhr.upload) {
                // 可在此处实现上传进度追踪
            }
            return myxhr;
        },
        success: function(response) {
            console.log("文件上传成功:", file.name, response);
            // 处理成功响应
        },
        error: function(jqXHR, textStatus, errorThrown) {
            console.error("文件上传失败:", file.name, textStatus, errorThrown);
            // 处理错误
        }
    });
}

<?php
header('Content-Type: application/json');

if (isset($_FILES['file']) && $_FILES['file']['error'] === UPLOAD_ERR_OK) {
    $fileTmpPath = $_FILES['file']['tmp_name'];
    $fileName = $_FILES['file']['name'];
    $fileSize = $_FILES['file']['size'];
    $fileType = $_FILES['file']['type'];

    // 确保上传目录存在且可写
    $uploadDir = 'uploads/';
    if (!is_dir($uploadDir)) {
        mkdir($uploadDir, 0777, true);
    }

    // 生成唯一的文件名以避免覆盖，这是处理并发上传的关键一步
    $newFileName = uniqid() . '_' . basename($fileName);
    $destPath = $uploadDir . $newFileName;

    if (move_uploaded_file($fileTmpPath, $destPath)) {
        echo json_encode([
            'status' => 'success',
            'message' => '文件上传成功',
            'fileName' => $newFileName,
            'originalName' => $fileName,
            'fileSize' => $fileSize,
            'filePath' => $destPath
        ]);
    } else {
        echo json_encode([
            'status' => 'error',
            'message' => '文件移动失败'
        ]);
    }
} else {
    // 处理各种上传错误
    $errorMessage = '文件上传失败。';
    if (isset($_FILES['file']['error'])) {
        switch ($_FILES['file']['error']) {
            case UPLOAD_ERR_INI_SIZE:
            case UPLOAD_ERR_FORM_SIZE:
                $errorMessage = '文件大小超出限制。';
                break;
            case UPLOAD_ERR_PARTIAL:
                $errorMessage = '文件只有部分被上传。';
                break;
            case UPLOAD_ERR_NO_FILE:
                $errorMessage = '没有文件被上传。';
                break;
            case UPLOAD_ERR_NO_TMP_DIR:
                $errorMessage = '缺少临时文件夹。';
                break;
            case UPLOAD_ERR_CANT_WRITE:
                $errorMessage = '文件写入失败。';
                break;
            case UPLOAD_ERR_EXTENSION:
                $errorMessage = 'PHP扩展阻止了文件上传。';
                break;
        }
    }
    echo json_encode([
        'status' => 'error',
        'message' => $errorMessage
    ]);
}
?>