query mysql database from inside a class

Question

I'm trying to run a query to a MySQL database from within a class and it's not working for some reason. I have the class in a separate file which I'm linking to with the require_once() function.

here's what the main .php file looks like:

<?php
  require_once("connect.php");
  require_once("theClass.php");

  $a = new theClass;
  $a->runQuery();
}

connect.php:

<?php
//connect to mySQL database
$mysqli = new mysqli("host", "user", "password", "db");
if ($mysqli->connect_errno)
{
    echo "<br><h1>Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error . "</h1><br>";
}

theClass.php:

<?php
require_once('connect.php');

class theClass
{
  //class variables and other functions here

  function runQuery()
  {
    $query = "SELECT col_1 FROM db.table";
    $stmt = $mysqli->prepare($query);
    stmt->execute();
    $stmt->bind_result($r);

    while($stmt->fetch())
    {
      echo $r . "<br>";
    }
  }
};

I've tried copying the class into the main .php file and it still doesn't work; however, I have used the exact same code( the query, prepare, execute, bind_result, and fetch part) in an external .php file as well as inside the main .php file and it has worked both times. This leads me to believe that you're not able to run queries from inside a class or that there is a different way of going about doing so. Could anyone point me in the right direction?

Thanks

It's worth noting that my connect.php file has the correct information and is working because I have been able to run queries in other .php files — Logan Besecker
– Logan Besecker, Commented Oct 19, 2012 at 23:50

kapa · Accepted Answer · 2012-10-19 23:57:32Z

9

Pass it to the method itself

You have to pass the database object to the method, because they are not in the same scope:

function runQuery($mysqli)

and call it like

$a = new theClass;
$a->runQuery($mysqli);

Pass it to the constructor

If your class makes a lot of database calls, you could simply pass it in the constructor and save it as a private variable for later use:

class theClass
{
  private $mysqli;

  function __construct($mysqli) {
    $this->mysqli = $mysqli;
  }

  function runQuery()
  {
    $query = "SELECT col_1 FROM db.table";
    $stmt = $this->mysqli->prepare($query);
    stmt->execute();
    $stmt->bind_result($r);

    while($stmt->fetch())
    {
      echo $r . "<br>";
    }
  }
};

and call it like

$a = new theClass($mysqli);
$a->runQuery();

Both methods make it clear that the dependency of your class is a mysqli object, which is good for future maintenance and readability.

edited Oct 19, 2012 at 23:57

answered Oct 19, 2012 at 23:52

kapa

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3 Comments

Logan Besecker Over a year ago

thanks, this works! Why is it that I don't need to pass the database object to an external function, but need to for classes?

kapa Over a year ago

@LoganBesecker External functions won't be able to use it either. Functions introduce a new variable scope.

Logan Besecker Over a year ago

really? somehow I've been able to get by without doing it this way in my external functions..

Omnikrys · Accepted Answer · 2012-10-19 23:52:31Z

2

You need to either pass $mysqli as a parameter or use global $mysqli

<?php
require_once('connect.php');

class theClass
{
  //class variables and other functions here

  function runQuery()
  {
    global $mysqli;
    $query = "SELECT col_1 FROM db.table";
    $stmt = $mysqli->prepare($query);
    stmt->execute();
    $stmt->bind_result($r);

    while($stmt->fetch())
    {
      echo $r . "<br>";
    }
  }
};

answered Oct 19, 2012 at 23:52

Omnikrys

2,55820 silver badges13 bronze badges

3 Comments

kapa Over a year ago

Globals will make your code smell quickly though, so for a better world, you should avoid using them.

Omnikrys Over a year ago

Just presenting options as it is a viable solution. It depends on how the app is structured as to which would work best.

kapa Over a year ago

Using classes and globals together does not make sense. Either write normal OOP or you can use the good old functions and save yourself a lot of trouble.

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Pass it to the method itself

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