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PHP 中文件上传的方法通常是通过 HTML 的表单来实现。上传文件时，需要使用 enctype="multipart/form-data" 属性来指定表单数据编码类型为multipart/form-data。

以下是一个简单的文件上传表单示例：

<form action="upload.php" method="post" enctype="multipart/form-data">
    Select image to upload:
    <input type="file" name="fileToUpload" id="fileToUpload">
    <input type="submit" value="Upload Image" name="submit">
</form>

在 upload.php 文件中，可以使用 $_FILES 超全局数组来获取上传的文件信息。以下是一个简单的文件上传处理示例：

<?php
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file, PATHINFO_EXTENSION));

// Check if file already exists
if (file_exists($target_file)) {echo "Sorry, file already exists.";
    $uploadOk = 0;
}

// Check file size
if ($_FILES["fileToUpload"]["size"] > 500000) {echo "Sorry, your file is too large.";
    $uploadOk = 0;
}

// Allow certain file formats
if ($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
    && $imageFileType != "gif") {echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
    $uploadOk = 0;
}

// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) {echo "Sorry, your file was not uploaded.";
// if everything is ok, try to upload file
} else {if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {echo "The file ". htmlspecialchars(basename($_FILES["fileToUpload"]["name"])). " has been uploaded.";
    } else {echo "Sorry, there was an error uploading your file.";
    }
}
?>

以上代码会将上传的文件保存到 uploads/ 目录，并输出上传成功或失败的信息。在实际项目中，还可以添加更多的文件验证和安全检查。

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