When debugging a small program that uploads files and stores them in the database, problems always occur.

When debugging a small program that uploads files and stores them in the database, problems always occur. Ask the experts here! In fact, this function should be very simple. I can see a program on the Internet and debug it myself, but there are unexpected errors. please kindly advise. Html front-end code: & lt; html & gt; & lt; body & gt; & lt; for always encounters problems when debugging a small program that uploads files and stores them in the database, ask the experts here!
In fact, this function should be very simple. I can see a program on the Internet and debug it myself, but there are unexpected errors. please kindly advise.

Html front-end code:

Php back-end code:

 if ($_FILES ['file'] != "none" && $_FILES ['file'] != "") 
{
$time_limit = 60;
set_time_limit ( $time_limit );

$file_type = $_FILES ['file'] ['type'];
$file_name = $_FILES ['file'] ['name'];
$file_size = $_FILES ['file'] ['size'];
$fp = fopen ( $_FILES ['file'] ['tmp_name'], "rb" );

if (! $fp)
{
die ( "file open error" );
}

$file_data = file_get_contents($_FILES["file"]['tmp_name']);
//$file_data = fread($fp, $file_size ) ;
fclose ( $fp );
 $conn = mysqli_connect ( "localhost", "root", "root" );
    if (! $conn)
    {
die ( "error : mysql connect failed" );
    }
    mysqli_select_db($conn,"onlytest" );
    $sql = "insert into onlytest.file (contents,type,name,size) values ('$file_data','$file_type','$file_name','$file_size')";
    $result = mysqli_query ($conn, $sql);
    //$id = mysqli_insert_id ($conn);
    //echo $id;
    mysqli_close ( $conn );
    set_time_limit ( 30 ); 
    if ($result)
    echo "succeed to upload the files ";
    else
    echo "failed!";
   
} 
else 
{
echo "You haven't uploaded any files!";
}
?>

The database is onlytest, the table name is file, and several keywords are id, contents, type, name, and size. The settings are as follows:

------ Solution --------------------
What is the error message?
------ Solution --------------------
What is the maximum size of Upload allowed for your configuration?
------ Solution --------------------
What are SQL statements that can be printed?
------ Solution --------------------
Filter it out. Add $ file_data to the mysql_escape_string function.