Fatal error: Call to a member function query() PHP CLASS

Question

I'm trying to write this class to connect and query my database, but I got this error:

Fatal error: Call to a member function query() on null in C:\xxxxxx\xxxx\xxxxx\xxxxxxx\pages\config\class.php on line 24

Php Code:

<?php

class Db{

    private static $db_host = "localhost";
    private static $db_user = "root";
    private static $db_pass = "";
    private static $db_name = "sivi";

    public $connection;

        public function db_connect() {    

            $connection = mysqli_connect(self::$db_host, self::$db_user, self::$db_pass, self::$db_name) or die("Error " . mysqli_error($connection)); 
            echo "Conexión realizada". "<br>";

    }  

    public function db_query($query){

          $connection = $this->db_connect();
          var_dump($query);
          $result = $connection->query($query);
          while($row = mysqli_fetch_array($result)) { 
          echo $row["COD_PRE"] . "<br>";
          }

    } 

}

$con = new Db();
$con->db_query('SELECT `COD_PRE`, `CODE` FROM `test` WHERE `CODE` = 457 AND CONFIN = 1');

?>

Answer 1

Your method:

public function db_connect() {    

            $connection = mysqli_connect(self::$db_host, self::$db_user, self::$db_pass, self::$db_name) or die("Error " . mysqli_error($connection)); 
            echo "Conexión realizada". "<br>";

    }  

does not retun the $connection back from the method so the rest method calls fail do this:

public function db_connect() {    

            $connection = mysqli_connect(self::$db_host, self::$db_user, self::$db_pass, self::$db_name) or die("Error " . mysqli_error($connection)); 
            echo "Conexión realizada". "<br>";
           return $connection; // return the $connection object
    }  

As mentioned, your code is not efficient since for every query performed the code (re-)connects to the DB. THis is unnecessarily expensive/inefficient.

There are many approaches to solve this.

1. Connect to the DB on the instantiation of the DB class

e.g

class Db{

    private static $db_host = "localhost";
    private static $db_user = "root";
    private static $db_pass = "";
    private static $db_name = "sivi";

    public $connection;

    public function __construct()
   {
      $this->connection = $this->db_connect();
   }  

    public function db_connect() {    

            $connection = mysqli_connect(self::$db_host, self::$db_user, self::$db_pass, self::$db_name) or die("Error " . mysqli_error($connection)); 
            echo "Conexión realizada". "<br>";
            return $connection;
    }  
}

2. Lazy connection, i.e connect only on first executed query

e.g

class Db{

    private static $db_host = "localhost";
    private static $db_user = "root";
    private static $db_pass = "";
    private static $db_name = "sivi";
    public $connection = null;
    public function __construct()
    {
    }  
    public function db_connect() {    
        $connection = mysqli_connect(self::$db_host, self::$db_user, self::$db_pass, self::$db_name) or die("Error " . mysqli_error($connection)); 
        echo "Conexión realizada". "<br>";
        return $connection;
    }  
    public function db_query($query){
        if ( null ==== $this->connection ) $this->connection = $this->db_connect();
        var_dump($query);
        $result = $this->connection->query($query);
        while($row = mysqli_fetch_array($result)) { 
            echo $row["COD_PRE"] . "<br>";
        }
    }
}

Answer 2

You have to use $this to access properties of the own class.

$this->connection = ....

$result = $this->connection->...

It would be better if you used a constructor to initiate the connection, currently you are opening a new connection to the database every time you use the query method.

edit: Your class could look like this

<?php

class Db {

    private $db_host = "localhost";
    private $db_user = "root";
    private $db_pass = "";
    private $db_name = "sivi";
    private $connection;

    public function __construct() {
        $this->connection = mysqli_connect($this->db_host, $this->db_user, $this->db_pass, $this->db_name) 
                or die("Error " . mysqli_error($this->connection));
    }

    public function db_query($query) {
        var_dump($query);
        $result = $this->connection->query($query);
        while ($row = mysqli_fetch_array($result)) {
            echo $row["COD_PRE"] . "<br>";
        }
    }

}

$con = new Db();
$con->db_query('SELECT `COD_PRE`, `CODE` FROM `test` WHERE `CODE` = 457 AND CONFIN = 1');
?>

I see no need for the properties to be static, so I changed them too.

The db_query method is in my opinion too unflexible, as it directly outputs your result. I would use fetch_all to return the whole resultset as an array. That way you can freely choose how you want to handle your results.