php上传下载怎么用

PHP有内置的上传文件函数和下载文件函数，可以使用以下函数来完成相关操作：

1.上传文件：

使用PHP的$_FILES超全局变量来获取上传文件的信息，然后使用move_uploaded_file()函数将文件从临时目录移动到目标目录。

<form action="upload.php" method="post" enctype="multipart/form-data">
    <input type="file" name="fileToUpload">
    <input type="submit" value="Upload File" name="submit">
</form>

<?php
if(isset($_POST["submit"])) {
    $target_directory = "uploads/";
    $target_file = $target_directory . basename($_FILES["fileToUpload"]["name"]);
    $uploadOk = 1;
    $imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));

    // Check if image file is a actual image or fake image
    if(isset($_POST["submit"])) {
        $check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
        if($check !== false) {
            echo "File is an image - " . $check["mime"] . ".";
            $uploadOk = 1;
        } else {
            echo "File is not an image.";
            $uploadOk = 0;
        }
    }

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

    // Check if file already exists
    if (file_exists($target_file)) {
        echo "Sorry, file already exists.";
        $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 ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
        } else {
            echo "Sorry, there was an error uploading your file.";
        }
    }
}
?>

2.下载文件：

使用PHP的readfile()函数来读取文件并输出到浏览器端，浏览器会自动弹出下载提示框，让用户选择是否下载文件。

<?php
$file = "example.jpg";

if (file_exists($file)) {
    header("Content-Description: File Transfer");
    header("Content-Type: application/octet-stream");
    header("Content-Disposition: attachment; filename="".basename($file).""");
    header("Expires: 0");
    header("Cache-Control: must-revalidate");
    header("Pragma: public");
    header("Content-Length: " . filesize($file));
    readfile($file);
    exit;
}
?>

在上面的示例中，我们设置了文件名和Content-Type头来确保浏览器显示正确的文件名，并知道如何处理文件。文件内容使用readfile()函数读取和输出到浏览器。