PHP 文件上传

In this tutorial you'll learn how to upload a file to the remote web server with PHP.

## Uploading Files with PHP
In this tutorial we will learn how to upload files on remote server using a Simple HTML form and PHP. You can upload any kind of file like images, videos, ZIP files, Microsoft Office documents, PDFs, as well as executables files and a wide range of other file types.

### Step 1: Creating an HTML form to upload the file
The following example will create a simple HTML form that can be used to upload files.

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>File Upload Form</title>
</head>
<body>
    <form action="upload-manager.php" method="post" enctype="multipart/form-data">
        <h2>Upload File</h2>
        <label for="fileSelect">Filename:</label>
        <input type="file" name="photo" id="fileSelect">
        <input type="submit" name="submit" value="Upload">
        <p><strong>Note:</strong> Only .jpg, .jpeg, .gif, .png formats allowed to a max size of 5 MB.</p>
    </form>
</body>
</html>
```
<div class="callout callout-info mb-3">Note: In addition to a file-select field the upload form must use the HTTP post method and must contain an enctype="multipart/form-data" attribute. This attribute ensures that the form data is encoded as mulitpart MIME data — which is required for uploading the large quantities of binary data such as image, audio, video, etc.</div>

### Step 2: Processing the uploaded file
Here's the complete code of our "upload-manager.php" file. It will store the uploaded file in a "upload" folder on permanent basis as well as implement some basic security check like file type and file size to ensure that users upload the correct file type and within the allowed limit.

```php
<?php
// Check if the form was submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){
    // Check if file was uploaded without errors
    if(isset($_FILES["photo"]) && $_FILES["photo"]["error"] == 0){
        $allowed = array("jpg" => "image/jpg", "jpeg" => "image/jpeg", "gif" => "image/gif", "png" => "image/png");
        $filename = $_FILES["photo"]["name"];
        $filetype = $_FILES["photo"]["type"];
        $filesize = $_FILES["photo"]["size"];
    
        // Verify file extension
        $ext = pathinfo($filename, PATHINFO_EXTENSION);
        if(!array_key_exists($ext, $allowed)) die("Error: Please select a valid file format.");
    
        // Verify file size - 5MB maximum
        $maxsize = 5 * 1024 * 1024;
        if($filesize > $maxsize) die("Error: File size is larger than the allowed limit.");
    
        // Verify MYME type of the file
        if(in_array($filetype, $allowed)){
            // Check whether file exists before uploading it
            if(file_exists("upload/" . $filename)){
                echo $filename . " is already exists.";
            } else{
                move_uploaded_file($_FILES["photo"]["tmp_name"], "upload/" . $filename);
                echo "Your file was uploaded successfully.";
            } 
        } else{
            echo "Error: There was a problem uploading your file. Please try again."; 
        }
    } else{
        echo "Error: " . $_FILES["photo"]["error"];
    }
}
?>
```
<div class="callout callout-info mb-3">Note: The above script prevents uploading a file with the same name as an existing file in the same folder. However, if you want to allow this just prepend the file name with a random string or timestamp, like $filename = time() . '_' . $_FILES["photo"]["name"];</div>

You might be wondering what this code was all about. Well, let's go through each part of this example code one by one for a better understanding of this process.

## Explanation of Code
Once the form is submitted information about the uploaded file can be accessed via PHP superglobal array called `$_FILES`. For example, our upload form contains a file select field called photo (i.e. `name="photo"`), if any user uploaded a file using this field, we can obtains its details like the name, type, size, temporary name or any error occurred while attempting the upload via the `$_FILES["photo"]` associative array, like this:

- `$_FILES["photo"]["name"]` — This array value specifies the original name of the file, including the file extension. It doesn't include the file path.
- `$_FILES["photo"]["type"]` — This array value specifies the MIME type of the file.
- `$_FILES["photo"]["size"]` — This array value specifies the file size, in bytes.
- `$_FILES["photo"]["tmp_name"]` — This array value specifies the temporary name including full path that is assigned to the file once it has been uploaded to the server.
- `$_FILES["photo"]["error"]` — This array value specifies error or status code associated with the file upload, e.g. it will be 0, if there is no error.

The PHP code in the following example will simply display the details of the uploaded file and stores it in a temporary directory on the web server.

```php
<?php
if($_FILES["photo"]["error"] > 0){
    echo "Error: " . $_FILES["photo"]["error"] . "<br>";
} else{
    echo "File Name: " . $_FILES["photo"]["name"] . "<br>";
    echo "File Type: " . $_FILES["photo"]["type"] . "<br>";
    echo "File Size: " . ($_FILES["photo"]["size"] / 1024) . " KB<br>";
    echo "Stored in: " . $_FILES["photo"]["tmp_name"];
}
?>
```
<div class="callout callout-success mb-3">Tip: Once a file has been successfully uploaded, it is automatically stored in a temporary directory on the server. To store this file on a permanent basis, you need to move it from the temporary directory to a permanent location using the PHP's `move_uploaded_file()` function.</div>