PHP constructors and static functions

Question

I'm a bit confused on how constructors work in PHP.

I have a class with a constructor which gets called when I instantiate a new object.

$foo = new Foo($args);

__construct($params) is called in the class Foo and it executes the appropriate initialization code.

However when I use the class to call a static function, the constructor is called again.

$bar = Foo::some_function(); //runs the constructor from Foo

This causes the constructor to execute, running the object initialization code that I intended only for when I create a new Foo object.

Am I missing the point of how constructors work? Or is there a way to prevent __construct() from executing when I use the class to make static function calls?

Should I use a "factory" function instead to do the object initialization? If so, what's the point of the constructor then?

::EDIT:: I have a form where users can upload photos to an album (create_photo.php) and an area where they can view the album (view_photos.php). Upon form submit:

$photo = new Photo($_FILES['photo'], $_POST['arg1'], ect..);

The Photo constructor creates and saves the photo. However in view_photo.php, when I call:

$photo = Photo::find_by_id($_POST['id']) //user-defined function to query database

This is causing Photo's constructor to run!

Answer 1

I see nothing that replicates your question.

See Demo: http://codepad.org/h2TMPYUV

Code:

class Foo {
    function __construct(){ 
        echo 'hi!';
    }
    static function bar(){
        return 'there';
    }
}

echo Foo::bar(); //output: "there"

Answer 2

Assumption PHP 5.x

Different goals, different path

1. create a new instance of a class (object)

   class myClassA
   {
      public $lv;
   
      public function __construct($par)
      {
          echo "Inside the constructor\n";
          $this->lv = $par;
      }
   }
   
   $a = new myClassA(11);
   $b = new myClassA(63);
   

   because we create a new object PHP calls:

   __construct($par);

   of the new object, so:

   $a->lv == 11 
   
   $b->lv == 63
   

2. use a function of a class

   class myClassB
   {
       public static $sv;
   
       public static function psf($par)
       {
           self::$sv = $par;
       }
   }
   
   myClassB::psf("Hello!");
   $rf = &myClassB::$sv;
   myClassB::psf("Hi.");
   

   now $rf == "Hi."

   function or variabiles must defined static to be accessed by ::, no object is created calling "psf", the "class variable" sv has only 1 instance inside the class.

3. use a singleton created by a Factory (myClassA is above)

   class myClassC
   {
   
       private static $singleton;
   
       public static function getInstance($par){
   
           if(is_null(self::$singleton)){
   
               self::$singleton = new myClassA($par);
   
           }
   
           return self::$singleton;
   
       }
   
   }
   
   $g = myClassC::getInstance("gino");
   echo "got G\n";
   
   $p = myClassC::getInstance("pino");
   echo "got P\n";
   

Using the factory (getInstance) the first time we construct a new object having $par set to gino.

Using the factory the second time $singleton has already a value that we return. No new object is created (no __construct is called, less memory & cpu is used).

The value of course is an object instanceOf myClassA and don't forget:

myClassC::$singleton->lv == "gino"

Pay attention to singletons:

What is so bad about singletons?

http://www.youtube.com/watch?v=-FRm3VPhseI

By my answer I don't want promote/demote singleton. Simply from the words in the question, I made this calc:

"static"+"__construct"="singleton"!