To implement file uploading of PHP, you must first set the HTML form enctype to multipart/form-data and specify the POST submission method. The backend receives data through $_FILES. 1. The front-end uses the <input type="file"> control and ensure the correct configuration of the form; 2. PHP obtains file information through $_FILES and judges the request method; 3. Verify the file type, size and prevent duplicate names, such as limiting the jpg/png format and not exceeding 2MB, and rename it with uniqid(); 4. Use move_uploaded_file() to move the file to the target directory and set appropriate permissions; 5. Adjust the php.ini parameters such as upload_max_filesize, post_max_size and max_file_uploads to support large files or more uploads, and the restart service will take effect after modification.

File upload is a very common requirement in web development. PHP, as an old backend language, naturally supports file upload function. As long as you understand the basic process and pay attention to a few key points, it is not difficult to achieve.

1. Basic steps: From HTML to PHP processing

To implement file upload, you must first provide a <input type="file"> control on the front-end page, and enctype property of the form must be set to multipart/form-data , otherwise the uploaded data cannot be transmitted correctly.

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

In the backend (such as upload.php), PHP will receive the uploaded file information through the $_FILES hyperglobal variable. This array contains information such as file name, temporary path, size, type, and upload status.

For example:

 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    if (isset($_FILES['myfile'])) {
        $file = $_FILES['myfile'];
        // Process file logic}
}

2. Common verification before uploading: security cannot be ignored

If the content uploaded by users is not restricted, it may bring security risks, such as uploading executable scripts or excessively large files. So it is best to do a few basic verifications before uploading:

• File type limitation : Only images or specific formats are allowed, and can be judged by mime類型or extension.
• File size control : Check $file['size'] before using is_uploaded_file() and move_uploaded_file() functions.
• Prevent overwriting of duplicate names : It is recommended to rename uploaded files, such as generating new file names with timestamps or UUIDs.

For example, the limit can only be uploaded with jpg/png format and the size does not exceed 2MB:

 $allowedTypes = ['image/jpeg', 'image/png'];
if (!in_array($file['type'], $allowedTypes)) {
    echo "Not allowed file types";
    exit;
}

if ($file['size'] > 2 * 1024 * 1024) {
    echo "The file is too large, limited to 2MB";
    exit;
}

3. Move files and permission settings

The uploaded file is initially saved in the server's temporary directory, and it needs to be moved to the target location using the move_uploaded_file() function. This function is also safer because it checks if the file is actually uploaded by HTTP POST.

 $uploadDir = 'uploads/';
$newName = uniqid() . '_' . basename($file['name']);
$targetPath = $uploadDir . $newName;

if (move_uploaded_file($file['tmp_name'], $targetPath)) {
    echo "Uploaded successfully";
} else {
    echo "Upload failed";
}

It should also be noted that the target directory must have write permissions (usually 755 or 777), but do not give 777 permissions easily. You can consider using the method of having permissions for the server running account to solve the problem.

4. Server configuration related adjustments

Some default settings in PHP may affect file upload behavior, such as an error will be reported when uploading large files. You need to check and modify the following parameters in php.ini :

• upload_max_filesize : The maximum allowed upload file size, the default is 2M.
• post_max_size : The maximum capacity of POST data is generally a little larger than upload_max_filesize.
• max_file_uploads : The maximum number of files allowed to be uploaded per request, the default is 20.

After modifying, remember to restart the web service to make the configuration take effect.

Basically that's it. File upload looks simple, but there are many details, especially in terms of security. As long as you handle the three aspects of verification, permissions and configuration, most problems can be avoided.

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