/ Design Patterns / Creational patterns / Singleton

Singleton Design Pattern in PHP

In the singleton pattern a class can distribute one instance of itself to other classes.

<?php

/**
 * Singleton classes
 */
class BookSingleton {
    private $author = 'Gamma, Helm, Johnson, and Vlissides';
    private $title  = 'Design Patterns';
    private static $book = NULL;
    private static $isLoanedOut = FALSE;

    private function __construct() {
    }

    static function borrowBook() {
      if (FALSE == self::$isLoanedOut) {
        if (NULL == self::$book) {
           self::$book = new BookSingleton();
        }
        self::$isLoanedOut = TRUE;
        return self::$book;
      } else {
        return NULL;
      }
    }

    function returnBook(BookSingleton $bookReturned) {
        self::$isLoanedOut = FALSE;
    }

    function getAuthor() {return $this->author;}

    function getTitle() {return $this->title;}

    function getAuthorAndTitle() {
      return $this->getTitle() . ' by ' . $this->getAuthor();
    }
  }
 
class BookBorrower {
    private $borrowedBook;
    private $haveBook = FALSE;

    function __construct() {
    }

    function getAuthorAndTitle() {
      if (TRUE == $this->haveBook) {
        return $this->borrowedBook->getAuthorAndTitle();
      } else {
        return "I don't have the book";
      }
    }

    function borrowBook() {
      $this->borrowedBook = BookSingleton::borrowBook();
      if ($this->borrowedBook == NULL) {
        $this->haveBook = FALSE;
      } else {
        $this->haveBook = TRUE;
      }
    }

    function returnBook() {
      $this->borrowedBook->returnBook($this->borrowedBook);
    }
  }
 
/**
 * Initialization
 */

  writeln('BEGIN TESTING SINGLETON PATTERN');
  writeln('');

  $bookBorrower1 = new BookBorrower();
  $bookBorrower2 = new BookBorrower();

  $bookBorrower1->borrowBook();
  writeln('BookBorrower1 asked to borrow the book');
  writeln('BookBorrower1 Author and Title: ');
  writeln($bookBorrower1->getAuthorAndTitle());
  writeln('');

  $bookBorrower2->borrowBook();
  writeln('BookBorrower2 asked to borrow the book');
  writeln('BookBorrower2 Author and Title: ');
  writeln($bookBorrower2->getAuthorAndTitle());
  writeln('');

  $bookBorrower1->returnBook();
  writeln('BookBorrower1 returned the book');
  writeln('');

  $bookBorrower2->borrowBook();
  writeln('BookBorrower2 Author and Title: ');
  writeln($bookBorrower1->getAuthorAndTitle());
  writeln('');

  writeln('END TESTING SINGLETON PATTERN');

  function writeln($line_in) {
    echo $line_in.'<br/>';
  }
?>

Output

BEGIN TESTING SINGLETON PATTERN


BookBorrower1 asked to borrow the book
BookBorrower1 Author and Title: 
Design Patterns by Gamma, Helm, Johnson, and Vlissides


BookBorrower2 asked to borrow the book
BookBorrower2 Author and Title: 
I don't have the book


BookBorrower1 returned the book


BookBorrower2 Author and Title: 
Design Patterns by Gamma, Helm, Johnson, and Vlissides


END TESTING SINGLETON PATTERN

Code examples

Java Singleton in Java Singleton in Java
C++ Singleton in C++: Before and after Singleton in C++
PHP Singleton in PHP
Delphi Singleton in Delphi
Python Singleton in Python