So, here is what I’ve learned:

The table:

Suppose the structure of the MySQL table (say ‘file_table‘) is like this:

It has three fileds:

• id : the primary key, integer type with auto increment feature.
• content: the content of file, blob type.
• type: varchar type, the type of the file (e.g. doc, pdf etc.).


Upload:

I think you can create a HTML form with a input with file type. Its pretty simple. Just use this code to make a input field be able to upload file:

<input id="file_upload" name="file_upload" type="file" />

Now the backend coding. I’m using PHP for this.

You can get the temp file name (the name server assigned to your uploaded file) using this:

$tmpName = $_FILES['file_upload']['tmp_name'];

Now following code segment reads the content of the file to a variable:

$fp = fopen($tmpName, 'r');
$file_size =  filesize($tmpName);
$content = fread($fp, $file_size);
$content = addslashes($content);
fclose($fp);

if(!get_magic_quotes_gpc())
{
$fileName = addslashes($fileName);
}

To get the file type, we use this segment of code:

$type = get_file_extension($tmpName);

function get_file_extension($str)
{
$i = strrpos($str,".");
if (!$i) { return ""; }
$l = strlen($str) - $i;
$ext = substr($str,$i+1,$l);
return $ext;
}

Now use this query to insert the file to database:

$query = 'INSERT INTO file_table VALUES (" ' . $content . ' ", " ' . $type . ' ")';
$result = @mysql_query($query) or die(mysql_error());

That’s it. We’ve uploaded our file to the database.

Download:

We’ve uploaded the file. Now we need to download it sometime. Thats not that hard actually.

We’ll follow these stpes:

• Get the id of desired file
• Fetch the file from database on the basis of the id
• Make the file ready for download

Say, we get the id from URL. Then following code will make the file available for download:

$id    = $_GET['id'];
$query = "SELECT content, type FROM file_table WHERE id = '$id' ";

$result = mysql_query($query) or die(mysql_error());
list($file, $ftype) = mysql_fetch_array($result);

header("Content-length: $filesize");
header("Content-type: $ftype");
header("Content-Disposition: attachment; filename = FILENAME.$ftype");
echo $file;

Here we set the header with the file information. Make sure, nothing else is printed on the page before the header() call. Otherwise it will show an error and terminate. You can place your desired file name at the place of ‘FILENAME‘. Another thing is, connection with the database server is to be made before any database operation. I think you know the function:

mysql_connect();

If not can check this post: Basic PHP

Bookmark:

Target viewers are assumed as familiar with C/JAVA or other similar programming language. So I didn’t bother to clarify basic syntax.

Writing PHP code:

All your PHP content should be inside <? ?> tags. That is the file format should be:

<?
Your content here.
?>

Most of the functions in PHP are similar to C. So if you can code in C, you can code in PHP. There are simple exceptions as:

Variables:
PHP variables has no data type. All are same. A PHP variable starts with $ sign. An example is:

$var1 = "Something";

Above variable contains a string. So you can do any string operation over it, such as, strcat, strlen etc. Have you noticed? The string functions are similar to C.

Now if another 3 variables are like this:

$var2 = 3;
$var3 = 2;
And
$var 4 = $var2 + $var3;

Now $var4 contains 5 by adding above two variables’ contents.
You can even do:

$var4 = $var1 + $var2;

This would provide no error, but no result either as there is a string and a number you are adding.

Print and Handling String:
If you want to print something on your browser you have to write:

echo "String content what you wish to print";
echo 'And something more';

Notice that you can use either double quote (“) or single quote (‘) to define a string.
Now to attach a variable with a string, we use the dot (.).
Example:

echo 'A number is ' . $var2 . ' and is an odd number';
This will print:
A number is 3 and is an odd number
You can also do:
echo $var1 . ' is a string and ' . $var2 .' is a number';
Which will print:
Something is a string and 3 is a number

If you do:

$var5 = $var1 . $var2;

Then both the $var1 and $var2 will be treated as strings and $var5 will contain:

Something3

Writing Functions:

Its very easy. The syntax is:

function function_name(params)
{
Do something useful.
return result (optional).
}

Now some DB connection:

The My SQL database connection and manipulation is done using mysql_ functions.
Such as:
mysql_connect();
mysql_select_db();
mysql_query();
mysql_fetch_row();

A database connection we’ve done earlier in another project is like this:

$host= 'localhost';
$user = 'root';
$auth = 'password';
$db = 'mydb';
$msg = 'Database error occurred. Please inform the administrator.';
mysql_connect($host, $user, $auth);
@mysql_select_db($db) or die($msg);

Running a Query:

An example from previous project:

function db_select_by_id($table_name, $id)
{
$sql = 'SELECT * FROM '.$table_name.' WHERE '.$table_name.'_id = '.$id;
$result = @mysql_query($sql);
if($result)
{
return @mysql_fetch_object($result);
}
else
{
if($debug)
{
echo mysql_error();
}
return NULL;
}
}

So if we call the above function like this:
$result = db_select_by_id('user', 4);
The function will run the the query:
SELECT * FROM user WHERE user_id = 4;
And will return the result as object. So the $result will contain an object with the fetched row elements as object elements. We can access elements of $result as:
echo $result -> id;
echo $result -> name;
etc.
Notice that, here we are fetching only one row. This is because, there should be only one row as we are searching using the primary key (id). If you need multiple rows you can use:

function db_select_all($table_name)
{
$sql = 'SELECT * FROM '.$table_name; $result = @mysql_query($sql);
if($result)
{
return $result;
}
else
{
if($debug)
{
echo mysql_error();
}
return NULL;
}
}

Where we select all the rows from the table. Now if $result has the return value from above function, we can retrieve using following process:

while($row = @mysql_fetch_object($result))
{
echo $row -> id;
echo $row -> name;
}

This will fetch every row and will print them until end.
Have you noticed the @ sing before mysql functions? This is used so that any error message is not printed on the browser. You can remove them now to see error messages during development period.
Now you can make your own functions with adding some restrictions to the query…

A very good resource which helps me every time is: http://php.net

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