I have a problem with "ADD" and "INSERT" functions in database - php

Question

I wanna ask you, how can I fix this code? I have a problem with "ADD" and "INSERT" functions in database. I can only delete from database, but "add and insert" functions do nothing.
this is my about.php file.
$mode = 'add';
$about = '';
if(isset($_GET['edit']) && is_numeric($_GET['edit'])){
$sql = "SELECT * FROM `about_me` WHERE `about_me`.`id` =".$_GET['edit'];
$result_about = mysqli_query($conn, $sql);
if(mysqli_num_rows($result_about) == 1) {
$mode = 'edit';
$about = mysqli_fetch_assoc($result_about);
}
}
$apie = '';
if(isset($_POST["submit"])){
if(isset($_POST["apie"])){
$apie = trim($_POST["apie"]);
}
}
elseif($mode=='edit') {
$apie = $about['about'];
}
if($mode=='add') {
if(($apie!='')){
$sql = 'INSERT INTO about_me(about)
VALUES ("'.$apie.'")';
mysqli_query($conn, $sql);
header('Location:about.php');
die();
}
}
elseif($mode=='edit') {
if(($apie!='')){
$sql = "UPDATE `about_me` SET `about` = '".$apie."' WHERE `about_me`.`id` = ".$_GET['edit'];
mysqli_query($conn, $sql) ;
}
}
<..>
<input type = "text" name = "apie" value = "<?php echo $apie; ?>">
<br><br>
<input type = "submit" name = "submit" value = "Gerai">
<br><br>
I checked mysql error, with https://www.w3schools.com/php/func_mysqli_error.asp, and then it insert in my DB. I think there is code foult, but I don't know where.

Answer 1

You have a slight error with your first query, you can't use parentheses next to the database selector, I'd recommend creating a separate file with a database connection that you can refer to as $conn, your first query should look like this:
$sql = "INSERT INTO about_me
VALUES (?)";
Also, you should look into prepared statements for your queries, instead of inserting them directly you use a question mark? to which you bind the parameter/parameters. This helps prevent SQL injections!
Hope this helps!:)