A quick PHP example; how to implement an Enum. PHP lacks this useful feature so this is a bit of a hack to emulate an Enum.

Example below with an example of usage;

<?php

namespace Company\AppName\PackageName;

/**
 * SystemProperty is designed as an enum
 *
 * Example usage:
 * <code>
 * echo SystemProperty::A_PROPERTY_VALUE;
 * </code>
 *
 * @author rmurray
 */
final class SystemProperty {
    
    /**
     * The xxx property, I think you will like it.
     * 
     * @var string
     */
    const A_PROPERTY_VALUE = 'wow-this-is-amazing';
    
    
    /* block instantiation */
    protected function __construct() {}
}

A useful, perhaps, class for database connections using the PHP PDO library: PDOConnection.php

Singleton implementation with the class method getConnection() returning a \PDO connection with connection details;

<?php

namespace ACompany\AnAppName\Dao;

/**
 * PDOConnection is a singleton implementation.
 * getConnection() returning an instance of PDO connection.
 * 
 * <code>
 * Example usage:
 * 
 * $pdo = PDOConnection::instance();
 * $conn = $pdo->getConnection( 'dsn', 'username', 'password' );
 *    
 * $results = $conn->exec("SELECT * FROM Table");
 * 
 * </code>
 *
 * @author rmurray
 */
class PDOConnection {
    
    /**
     * singleton instance
     * 
     * @var PDOConnection 
     */
    protected static $_instance = null;

    /**
     * Returns singleton instance of PDOConnection
     * 
     * @return PDOConnection 
     */
    public static function instance() {
        
        if ( !isset( self::$_instance ) ) {
            
            self::$_instance = new PDOConnection();
            
        }
        
        return self::$_instance;
    }
    
    /**
     * Hide constructor, protected so only subclasses and self can use
     */
    protected function __construct() {}
    
    function __destruct(){}
    
    /**
     * Return a PDO connection using the dsn and credentials provided
     * 
     * @param string $dsn The DSN to the database
     * @param string $username Database username
     * @param string $password Database password
     * @return PDO connection to the database
     * @throws PDOException
     * @throws Exception
     */
    public function getConnection($dsn, $username, $password) {
        
        $conn = null;
        try {
            
            $conn = new \PDO($dsn, $username, $password);
            
            //Set common attributes
            $conn->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
            
            return $conn;
            
        } catch (PDOException $e) {
            
            //TODO: flag to disable errors?
            throw $e;
            
        }
        catch(Exception $e) {
            
            //TODO: flag to disable errors?
            throw $e;
            
        }
    }
    
    /** PHP seems to need these stubbed to ensure true singleton **/
    public function __clone()
    {
        return false;
    }
    public function __wakeup()
    {
        return false;
    }
}