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SQL Statement does not insert the required value to table column

I'm inserting some values to a table using a sql statement. All the values get inserted, except the 'qty' column. When I do an echo for sql statement, it shows the value has been assigned to the sql statement. Table column data type is INT. Can anyone help me to spot the problem?
$it_code = $_POST['id'];
//Prompt user to enter required qty
echo '<script>';
echo 'var qty = prompt("Enter Qty", "");';
echo '</script>';
//Get the item details from item table
$sqlString = "Select * from items where it_code='".$it_code."'";
$objItem = new Allfunctions;
$result = $objItem->retriveData($sqlString);
//Get the selected qty from java script to php variable
$qty = "<script>document.writeln(qty);</script>";
$user_id =$_SESSION['user_id'];
//Insert to orders table
$sqlStringInsert = "INSERT INTO orders(user_id,it_code,qty,status,tr_timestamp) VALUES ('$user_id','$it_code','$qty','PENDING',CURRENT_TIMESTAMP())";
echo $sqlStringInsert;
$objItem->addToTable($sqlStringInsert,'Succsessfully added.','Error adding'); // This is a custom built method to insert data in to a table by taking the sql statement as the parameter
Following is the sql statement generated,
Following is the table data. Qty is always 0 eventhood the sql statement had a value at qty column.

Obligatory warning
The code as you have shown it should not be used in production as it is vulnerable to SQL-injection attacks. Preventing this is well covered on this site in other answers.
Answer
The issue is that the value of $qty you are injecting into your SQL is the string "<script>document.writeln(qty);</script>". You just can't see it because you are likely echoing it out to the browser to test it.
Wrap your echoing of the SQL statement in a call to htmlentities() (docs) to see what's actually happening.
Depending on the version and settings, MySQL is very forgiving of bad data being injected to a column. In this case, it sees a string of text being inserted to a numeric column, and just truncates it to 0. See this in action here:
CREATE TABLE Foo (
Id INT,
Qty INT
);
INSERT INTO Foo(Id, Qty) VALUES (1, 'a string of text');
SELECT * FROM Foo; -- returns a row (1, 0)
https://www.db-fiddle.com/f/dEoaYGEyXEjs6ocVBwyyyr/1

increment value in database using PHP and AJAX

I am trying to make value +1 in database every time use use button.
my function in HTML:
function onClick(arg){
alert("thx fo click");
$.ajax({
type: 'POST',
url: 'data.php',
data: {
'arg': arg,
},
success: function(response) {
}
});
}
arg means value of button and it is a ID for a row in the database
and PHP:
<?php
$link = mysql_connect($servername, $username, $password);
$id = $_POST['arg'];
$sql = "UPDATE Buttons(SUMA) SET SUMA = SUMA + 1 WHERE ID = '$id'";
$conn->query($sql);
mysql_close($link);
?>
And that make nothing. How can i fix it?

You have several syntax errors here.
First and foremost though, check out mysqli_ (or PDO) and start using that instead of mysql_
For why to use mysqli_ - MySQL vs MySQLi when using PHP
Comparing mysqli_ and PDO - https://websitebeaver.com/php-pdo-vs-mysqli
With that out of the way....
You're defining your database connection without selecting a schema, but don't reference your schema in the query, meaning mysql won't know what to update. Either reference your schema in the connection or in each query. Also check on your table name, is it really Buttons(SUMA)?
You defined your database connection as $link, but are using $conn to attempt the query. Probably a 'typo' from copy and paste. Be careful of this...
As Artistic Phoenix mentioned, you have to make sure you're column cannot be set to NULL, and starts at 0 to begin. While you're at it and we're going through, make sure your datatype is set to int for the increment count.
After making those changes if you don't have success, I'd try running your query outside your code in a DB manager to ensure that portion is having the intended affect on your data, before looking at the errors in your code.
I'm guessing the arugment is passing correctly to your script, but to confirm, you can always echo it on the backend, and to be doubly sure alert() it in JS before it's passed through.
Take the time to go through that reading, update your script to use mysqli_ or PDO, and if you're still having troubles, I'm more than happy to jump back in here and help you further.

What is 1 + NULL it's still NULL.
IF you didn't default the column to '0' , then you can't increment it.
This can best be shown in a simple DB fiddle
Starting Null:
CREATE TABLE t(
id INT(10),
v INT(10)
);
INSERT INTO t (id)VALUES(1);
SELECT * FROM t;
UPDATE t SET v = v+1 WHERE id=1;
SELECT * FROM t;
In Both selects you will get a value of null for v as seen below in the fiddle:
https://www.db-fiddle.com/f/m1vgKpov1oiRJEfZEgmk1j/0
In simple terms, you cannot add 1 (or any number) to a NULL value. Well you can but it's still null. null + 1 = null
Starting 0:
CREATE TABLE t(
id INT(10),
v INT(10) default 0
);
INSERT INTO t (id)VALUES(1);
SELECT * FROM t;
UPDATE t SET v = v+1 WHERE id=1;
SELECT * FROM t;
In this case the first Select return 0 for v and the second returns 1 for v. As seen in the modified fiddle.
https://www.db-fiddle.com/f/m1vgKpov1oiRJEfZEgmk1j/1
Also (SQLInjection)
As I said in the comments:
What if arg = "' OR 1 --"
Or in other words don't inject user variables (or any clientside data) into your SQL or it winds up looking like this:
UPDATE `Buttons(SUMA)` SET SUMA = SUMA + 1 WHERE ID = '' OR 1 --'"
Which will increment every row in your DB that is not null. Basically the ' closes the other quote, then OR 1 is always true(for every row). Unfortinalty I cant show the -- comment part in the fiddle (it comments out the second select), but here is the end result.
https://www.db-fiddle.com/f/m1vgKpov1oiRJEfZEgmk1j/3
This is why we use prepared statements. This example is somewhat trivial but I have seen login code on here I was able to bypass simple by putting "' OR 1 LIMIT 1 --'" in, and with offset you could iterate though each user. They were looking for 1 row to be returned on the match of a username and password.
A few other things:
Table name Buttons(SUMA) is that really the name, as it will only work if escaped with the backtic. As I did in the above SQLInjection example.
$link = mysql_connect($servername, $username, $password); are these defined, the open tag is right above them. I generally chock that up to simplified example code. But it's worth asking. Obviously you can't connect to the DB if those are undefined.
Cheers!

Execute query on enter php page

I need a query to execute when entering a page (php). I've already added this to the top.
Code:-
mysql_query("INSERT INTO `table_name` (`player_id`, `unit1`, `unit2`, `unit3`, `unit4`)
VALUES ('".$_SESSION['user']['id']."', 0, 0, 0, 0)");
When my users enter the page that contains that, the query should be executed. Yes database connection is included.
How do I fix this? (also will it duplicate player_id)?

First mysql is deprecated use mysqli.
Your query is fine how i can see, but your table could be a problem.
You should not have primary key on player_id if you wish to duplicate player_id. (Otherwise keep the primary key).
Also if you wish to have one player_id with those results (not multiple / duplicates), then you could create query in this way:
$query = "INSERT INTO `table_name` (`player_id`, `unit1`, `unit2`, `unit3`, `unit4`)
VALUES ('".$_SESSION['user']['id']."', 0, 0, 0, 0)
ON DUPLICATE KEY UPDATE `unit1`=0, `unit2`=0, `unit3`=0, `unit4`=0";
UPDATE:
Also there could be a problem that you are missing a database identifier at mysql_query:
mixed mysql_query ( string $query [, resource $link_identifier = NULL ] )
like mysql_query("MYQUERY", $mylinkISavedAtConnectingToDatabase);

Construct mySQL query from dynamic select boxes

I've already been able to construct a mySQL query using select boxes. Starting with ajax to PHP like so.
html/js (relevant parts)
var call = true;
switch(parseInt($(this).val())) {
case 1:
cat_code ="ager";
sortv = "database_percent";
sorto = "asc";
break;
case 2:
cat_code ="ager";
sortv = "database_percent";
sorto = "desc";
break;
default:
cat_code ="ager";
sortv = "value";
sorto = "asc";
break;
}
if(call) {
$.ajax({
url: 'all_get_2.php',
type: 'GET',
dataType: 'JSON',
data: {cat_code: cat_code, sortvalue: sortv ,sortorder:sorto},
success: function(data) {
//stuff
}
})
PHP (relevant parts...just an example)
$whereCategory = isset($_GET['cat_code'])? "{$_GET['cat_code']}" : '';
$sortvalue = isset($_GET['sortvalue'])? "{$_GET['sortvalue']}" : '';
$sortorder = isset($_GET['sortorder'])? "{$_GET['sortorder']}" : '';
$sql = "select count(guid) from full_db2 where '{$whereCategory}' = '{$sortvalue}'' and example = '{$sortorder};";
$result = $dbh->query($sql)->fetchAll(PDO::FETCH_ASSOC);
header('Content-type: application/json');
echo json_encode($result);
All's well. I mixed up some variables in this illustration, but that's not the issue. This returns the count for the attributes that the user selected from the selects. The selects are dynamically generated based on what the user selected in the previous select.
What if I wanted users to be able to select a bunch more attributes to add to the query. So, for example, instead of finding the count for the number of entries in the database (users) who's age is more than 40, they could add on more to this like users with age greater than 40 and married and have children.
I considered basically adding an option to add another set of select boxes (e.g., click a "+" and new set appears), but I quickly realized that I wouldn't be able to figure out how many selects were out there and a logical way to create a query.
What's the best possible way to allow users to select a (theoretically) unlimited number of attributes to construct in an single mySQL query?

I would use the HTML <select> & <option> tags to present the various SQL conditional operators (AND, OR, >, <, =, !=) along with your field/table names, then concatenate them together to build your query.
As mentioned in the comments, you could utilize the array capabilities like name="checkbox[]" and submit it as an HTML form (POST or GET) to simplify concatenation.
You will want to escape and sanitize anything sent to the database to avoid SQL injection as HTML pages can be edited client side. Without sanitization your entire database will be accessible to a savvy user. This can be as simple as verifying that the array contents matches actual table names or one of the conditional operators and then discarding if it doesn't match. Escaping using mysqli::escape_string (http://php.net/manual/en/mysqli.real-escape-string.php) is also a good idea.

I ended up having to redo this question after I was able to figure out a few things. Here's how I progressed and check the accepted answer to see how it all is done together:
Create array for PDO from variables passed from jquery

Passing Mysql data to PHP array then to javascript

Let me start by explaining the situation:
I have a MySql table that contains several columns, of which a user id, a race id, a lap time, a lap number and I want to put this information into an array in PHP which I will then send to a java script.
My JavaScript array should end up looking like this :
first row:
[laptime1 user1, laptime2 user1, laptime3 user1,...]
second row:
[laptime1 user2, laptime2 user2, laptime3 user2,...]
Here's my current situation:
I first tried to test this situation for a single user and ran into lots of problems because my lap times in MySql are floats and when using json_encode it turned everything into strings, which did not work for my javascript as it started outputting the wrong values.
For example:
The first value was "8" instead of "84,521", then the second value was "4", etc..)...
Sadly, I found a potential solution with the numeric check option, but cannot use it as my hosting runs a PHP version that doesn't support it.
So I found the following solution, which I fiddled with a bit and that works for a single user (it might look messy to you, I'm really a beginner and punching above my weight, but it works) :
$query = doquery("SELECT racelaptime,userid FROM {{table}} WHERE raceid='1' ORDER BY racelap", "liverace");
while(($result = mysql_fetch_array($query))) {
$data[] = (float)$result['racelaptime'];
}
$script = $script . "\nvar myArray = new Array(";
foreach ($data as $key => $value){
if ($key < (count($data)-1)){
$script = $script . $value . ',';
}
else {
$script = $script . $value . ");\n";
}
}
This outputs an array in JavaScript that looks like this :
myArray=[84.521,83.800,81.900]
Which is great, as this is exactly what my java script requires as input (time in seconds, separated by commas for each lap).
Now I would like to implement the multiple user element but I'm stumped as to how I can work that out...
My MySQL query is still sorted by race lap but I also kind of need to sort the data by user id as I want all the laps of each user sorted in 1 row, Also, the user id is unknown to me and can vary (depends which user posts the time) so I can't really do a "if userid==1 save this here and then go to next one".
Should I use a foreach statement in the while loop that stores the data, but how can I tell him to store all the laps by the same user in the first row (and the next user in the second row, etc...) without using tons of SQL queries ?
If you can offer a more elegant solution than my current one for passing the PHP array to JavaScript, I would be more than happy to make changes but otherwise a simple solution using the current "setup" would be great too (hope it's all clear enough).
Any help would be very much appreciated, thanks in advance !

For multiple user element I would use a multidimensional array >
$query = doquery("SELECT racelaptime,userid FROM {{table}} WHERE raceid='1' ORDER BY racelap", "liverace");
// Loop the DB result
while(($result = mysql_fetch_array($query))) {
// Check if this ID is already in the data array
if(!array_key_exists($result['userid'], $data)){
// Create array for current user
$data[$result['userid']] = array();
}
// Add the current race time to the array (do not need to use the float)
$data[$result['userid']][] = $result['racelaptime'];
}
// Echo json data
echo json_encode($data);
Now what you need to do on the Javascript side when handling this array is to go through each of the user
$.each(data, function(key, value){
// Use parseFloat to keep the decimal value
alert(key + ' Has the following values - ' + value);
// If you want to display the racing values you simply
$.each(value, function(k, parseFloat(v)){
alert(v);
})
})
Is this what you needed or am I completely out of the scope?