Weedpacket Adding mysqli_error(db::$db) removed the warnings. But the query is wrong. It says: You have an error in your SQL syntax ... right syntax to use near '?,?,?, NOW() )' at line 7

      $sql = "INSERT INTO transactions_in(
		        transact_in_date,
			transact_in_note,
			transact_in_by_user,
			transact_in_date_added
			)
			VALUES (?,?,?,
			 NOW()
			)";

 db::query($sql,[$transact_date, $post['transact_note'], $_SESSION['user_name']]);

//From this class rather inside final class db{}
static function query($sql)
{


	$result = mysqli_query(self::$db, $sql);
	if(!$result) trigger_error("MySQL UPDATE error: ".mysqli_error(db::$db).'<pre>'.$sql.'</pre><br>');
	#if($_SESSION['access'] == 'admin') recache($sql);
}

Do you think it there's wrong with the prepared statement?

Thank you for your time, helping me here.

pbismad $sql = "INSERT INTO transactions_in(
transact_in_date,
transact_in_note,
transact_in_by_user,
transact_in_date_added
)
VALUES (?,?,?,
NOW()
)";

db::query($sql,[$transact_date, $post['transact_note'], $_SESSION['user_name']]);

I got an error using this code. And I mistakenly paste the wrong code here. Sorry, it should be.

  db::select($sql);

and I am using this class inside final class db{}:

 static function select($sql, $args=false){
	
	if($args['pager']){
		$limit = self::load_pager($args['pager']);
		$sql .= 'LIMIT '.$limit;
	}
	
	$result = mysqli_query(self::$db, $sql);
	if(!$result) trigger_error("MySQL SELECT error: ".mysqli_error()." using ".$sql); //error 1 

	if(@$args['explain']==true) self::explain($sql);
	
	while ($r = mysqli_fetch_array($result,MYSQLI_ASSOC)){  //error 2
			$row[] = $r;
	}

	mysqli_free_result($result); //error 3

	@array_walk_recursive($row, 'de_clean_post_recursive');
	return @$row;
  }

So now my latest code:

    $sql = "INSERT INTO transactions_in(
			        transact_in_date,
				transact_in_note,
				transact_in_by_user,
				transact_in_date_added
				)
				VALUES (?,?,?,
				 NOW()
				)";

     db::select($sql,[$transact_date, $post['transact_note'], $_SESSION['user_name']]);

And got this error:
Error 1: mysqli_error() expects exactly 1 parameter, 0 given ...
Error 2: mysqli_fetch_array() expects parameter 1 to be mysqli_result, object given ...
Error 3: mysqli_free_result() expects parameter 1 to be mysqli_result, boolean given ...

Thank you for the concern, I am developing the program on my local machine. So nothing to worry about the errors.

I am on my way re-coding my old code using mysql function which is already deprecated.
I am using a class to connect on my database:

<?php
    
final class db{
     static $db, $pager, $pager_vars;

static function config($conf=false){

	self::$db = mysqli_connect($conf['host'], $conf['username'], $conf['password']);
    if (!self::$db) {
        die("Database connection failed: " . mysqli_connect_error());
    }

    mysqli_select_db(self::$db, $conf['database']);

  }
}?>

//This is how I use a query

<?php
     $sql = "INSERT INTO transactions_in(
				        transact_in_date,
					transact_in_note,
					transact_in_by_user,
					transact_in_date_added
					)
					VALUES(
					'{$transact_date}',
					'{$post['transact_note']}',
					'{$_SESSION['user_name']}',
					 NOW()
					)";

  db::query($sql);
  mysqli_close(); //close the connection

}

?>

1. Am I doing the right way? I used mysqli_close(); to disconnect. But I have an error: "mysqli_close() expects exactly 1 parameter, 0 given..."
   What I have tried is to add the connection to my code e.g (mysqli_close(db::config($db))); but I got the same error.

2. And is there a way I can prevent an SQL injection on my code? Like using mysqli_real_escape_string? How to do it on my code structure?

laserlight Error, it is not connecting to the database. Do you have any guide where I can begin with this (maybe procedural approach)? I am a beginner, but I want this old program to be upgraded.

I need help converting mysql to msqli in the program I am using.
This is the database class to connect to database and to insert,select,delete and update.

<?php
final class db{
static $db;

static function config($conf=false){
	#echo '<pre>'.print_r($conf, 1).'</pre>';
	self::$db = mysql_connect($conf['host'], $conf['username'], $conf['password']);
	mysql_select_db($conf['database'], self::$db);
}

static function changedb($database){
	$resp = mysql_select_db($database);
	return($resp);
}

static function select($sql, $args=false){
	
	if($args['pager']){
		$limit = self::load_pager($args['pager']);
		$sql .= 'LIMIT '.$limit;
	}
	
	$result = mysql_query($sql, self::$db);
	if(!$result) trigger_error("MySQL SELECT error: ".mysql_error()." using ".$sql);

	if(@$args['explain']==true) self::explain($sql);
	
	while ($r = mysql_fetch_array($result,MYSQL_ASSOC)){
			$row[] = $r;
	}


	mysql_free_result($result);

	@array_walk_recursive($row, 'de_clean_post_recursive');
	return @$row;
}

static function select_row($sql, $args=false){
		$result = mysql_query($sql, self::$db);
		if(!$result) trigger_error("MySQL SELECT error: ".mysql_error()." using ".$sql);
		
		if(@$args['explain']==true) self::explain($sql);
		
		$row = mysql_fetch_array($result,MYSQL_ASSOC);			
		mysql_free_result($result);

		@array_walk_recursive($row, 'de_clean_post_recursive');					
		return @$row;
}

static function query($sql)
{
	$result = mysql_query($sql, self::$db);
	if(!$result) trigger_error("MySQL UPDATE error: ".mysql_error().'<pre>'.$sql.'</pre><br>');
	#if($_SESSION['access'] == 'admin') recache($sql);
}

static function explain($sql){
	
	trigger_error("SQL: ".$sql);
	$explain = mysql_query("EXPLAIN {$sql}", self::$db);
	if ($row = mysql_fetch_assoc($explain)) { 
		foreach ($row as $key => $value) { 
			trigger_error("$key: $value\n"); 
		}
	} 

}

}
?>

USAGE:
I use this to select a row:
<?
$sql = "SELECT * FROM user
WHERE userid = {$_SESSION['userid']}";
$user = db::select_row($sql);
?>

And this to select multiple rows
<?
$sql = "SELECT * FROM user";
$users = db::select($sql);
?>

To update:
<?
$sql = "UPDATE user SET user_name = '".($post['user_name'])."'
WHERE userid = '{$_SESSION['userid']}' LIMIT 1";
db::query($sql);
?>

To insert:
<?
$sql = "INSERT INTO user( ...,..,...,..) VALUES(..,..,..,..')";
db::query($sql);
?>

To delete:
<?
db::query('DELETE FROM user WHERE userid = "'.intval($GET['id']).'" LIMIT 1');
?>
OR
<?
$sql = "DELETE FROM user WHERE userid = "'.intval($GET['id']).'" LIMIT 1";
db::query($sql);
?>

I tried changing all mysql to mysqli but I can't connect to the server. Thank you for any help.