using global DB variable inside classes in PHP

Question

How can I use global DB variable inside class? Let's say I have this in my config.php

$dbh = new PDO("mysql:host=localhost;dbname=mydb", "root", "");

and I want to use this $dbh inside class as follows (MyClass.php)

class MyClass
{
   public function DoSomething($plogin_id)
   {
        $sql = "SELECT * FROM mytable WHERE login_id = :login_id";      
        $stmt = $dbh->prepare($sql);    //line 14
        $stmt->bindParam(':login_id', $plogin_id, PDO::PARAM_STR);
   }
}

And inside my index.php file I am using this MyClass as follows:

include "config.php";
$MyObject = new MyClass();
$login_result = $MyObject->DoSomething("admin");

It is giving me error:

Fatal error: Call to a member function prepare() on a non-object in C:\xampp\htdocs\MyProject\admin\includes\classes\MyClass.php on line 14

Jan Hančič · Accepted Answer · 2010-12-16 08:33:15Z

7

You have to pass the $dbh object to MyClass somehow. Since you don't want to use globals, I suggests you pass it to MyClass's constructor.

Your MyClass and index.php could look something like this:

class MyClass
{
   protected $dbh = null;

   public function __construct ( PDO $Pdh )
   {
      $this->dbh = $Pdh;
   }

   public function DoSomething($plogin_id)
   {
        $sql = "SELECT * FROM mytable WHERE login_id = :login_id";      
        $stmt = $this->dbh->prepare($sql);    //line 14
        $stmt->bindParam(':login_id', $plogin_id, PDO::PARAM_STR);
   }
}

// in your index.php
$MyObject = new MyClass ( $dbh );

This is basically a pattern called dependency injection. See this excellent tutorial for more info on what this is.

answered Dec 16, 2010 at 8:33

Jan Hančič

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Comments

Spiny Norman · Accepted Answer · 2010-12-16 08:34:33Z

You can introduce it into your function by using:

global $dbh;

However, it might be a better idea to add it to the class, like this:

class MyClass
{
    private $dbh;

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

    public function DoSomething($plogin_id)
    {
        $sql = "SELECT * FROM mytable WHERE login_id = :login_id";      
        $stmt = $this->dbh->prepare($sql);    //line 14
        $stmt->bindParam(':login_id', $plogin_id, PDO::PARAM_STR);
    }
}

and then:

include "config.php";
$MyObject = new MyClass($dbh); // I'm assuming $dbh is created in config.php
$login_result = $MyObject->DoSomething("admin");

Or, introduce it into your function at call time:

class MyClass
{
    public function DoSomething($plogin_id, $dbh)
    {
        $sql = "SELECT * FROM mytable WHERE login_id = :login_id";      
        $stmt = $dbh->prepare($sql);    //line 14
        $stmt->bindParam(':login_id', $plogin_id, PDO::PARAM_STR);
    }
}

And then:

include "config.php";
$MyObject = new MyClass($dbh); // I'm assuming $dbh is created in config.php
$login_result = $MyObject->DoSomething("admin", $dbh);

Alin P. · Accepted Answer · 2010-12-16 09:18:10Z

0

This is the most common method:

public function DoSomething($plogin_id){
    global $dbh;
    ...

Alternative approach:

Create a singleton for the database connection. And you use that instead of a global variable:

$stmt = MyDBConn::getInstance()->prepare($sql);

edited Dec 16, 2010 at 9:18

answered Dec 16, 2010 at 8:33

Alin P.

44.5k13 gold badges79 silver badges93 bronze badges

13 Comments

Jan Hančič Over a year ago

You are advocating the use of globals.

RageZ Over a year ago

@Alin; I didn't down vote but the problem you actually answer to the title of the question, but let's say the OP misunderstood the problem, his problem is actually not using this

Alin P. Over a year ago

Did you read the question?! This is the way to use global variables in a function. Weather it's a good or a bad thing it's a totally different issue.

Jan Hančič Over a year ago

Still, we should be promoting best practises here. Not layzines.

Alin P. Over a year ago

@Jan Hančič Best practices depend on the situation. If the user is a beginner and doesn't know about the global statement don't you think he should learn about that first before moving on to design patterns? This is how bad programmers are born... skipping the basics.

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