This post will show you how to create a PDO connection class using the singleton pattern. Before jumping into the code, take a look at the following paragraphs if you are new to PDO or the singleton pattern.
What is PDO?
PDO stands for PHP data objects, it is an API and can be used as an alternative to the MySQL, or MySQLi extensions. Using the API it is possible to connect to databases in a consistent way, meaning it is possible to switch between different types of databases easily. For example, switching from a MySQL database to a SQLite can be achieved by changing a single line of code.
The API also has a bunch of useful methods. An example being ‘prepare()’ and ‘execute()’ which make it possible to write prepared statements easily.
Check out the php.net page for further info.
What is the singleton pattern?
A design pattern is normally used as a solution to a common problem, it can be thought of as a template or blueprint on which code can be structured. The singleton pattern is used to ensure there is only one instance of a class, and that global access is possible. Although there are cases when connections to multiple databases will be required it is quite rare.
With that said, we can assume that creating multiple instances of the database connection class would be a wasteful use of resources. To prevent this, the following singleton class can be used.
The code
Below is the class, check the comments for explanations.
<?php
//db connection class using singleton pattern
class dbConn{
//variable to hold connection object.
protected static $db;
//private construct - class cannot be instatiated externally.
private function __construct() {
try {
// assign PDO object to db variable
self::$db = new PDO( 'mysql:host=localhost;dbname=DBNAME', 'USERNAME', 'PASSWORD' );
self::$db->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
}
catch (PDOException $e) {
//Output error - would normally log this to error file rather than output to user.
echo "Connection Error: " . $e->getMessage();
}
}
// get connection function. Static method - accessible without instantiation
public static function getConnection() {
//Guarantees single instance, if no connection object exists then create one.
if (!self::$db) {
//new connection object.
new dbConn();
}
//return connection.
return self::$db;
}
}//end class
?>Further explanation
A quick over-view of how the class works:
The ‘getConnection()’ method is called. This method checks that there is no existing connection, if this is the case then it creates a new instance of the ‘dbConn’ class (itself). Upon creation of this class the constructor is executed which then creates a new instance of the PDO class, this connects to the database. The PDO object is then saved to the ‘db’ property.
So, that’s the singleton pattern. It’s probably the easiest pattern to understand so i wont go into too much detail. It is worth noting that the constructor is set to private, this means the class cannot be instantiated without using the ‘getConnection()’ method. If the constructor was public, then it would be possible to create multiple instances of the class.
Example usage
<?php //calling static method ( '::' rather than '->') $db = dbConn::getConnection(); ?>
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