class scope, memory and other strange stuff

cioannou

As a newbie I tried to "google" around in order to understand how classes work in terms of scope, server memory, user session etc.

Unfortunately could not find any useful information, so here I am.

The problem is that I cannot understand how are classes handled.

Question 1:

e.g. (index.php)


echo "Hello World!";

var $sample;

$sample = &new MyClass($value1, $value2, $value3);

What happens if I refresh the page? Is a new class created everytime I refresh?

Question 2:

From the previous page I need to call another page (e.g. index2.php) and
use the class created in index.php, how can I do that?

Question 3:

In index2.php I create a new class

require_once ('test_class.php');

$testvar = &new TestClass($value1);

What happens with this class when the user returns to index.php, is "TestClass" still available somehow?

Qustion 4:
May I use "global" and "local" classes? e.g. MySQLClass is global for the whole site BUT "StringHandlerClass" is only locally (inside a function perhaps) used for a specific purpose and then must be removed from memory.

Thanks guys

sneakyimp

1) Technically, yes. PHP and apache do some caching and stuff but each time you refresh the page, the code executes all over again. If you are writing log files or creating database records, it will do that stuff all over again.

On the other hand, if it doesn't create any lasting db or log records, then the class you create should be pretty much the same each time if you call with the same values--unless you are using random functions or something like that. It all depends on your class definition.

2) if index2.php has code that outputs html, then you may or may not want to do this because it will spit out all kind of html, but if your index.php just runs some php code and doesn't output anything then give it a whirl:

include('index2.php');

3) now you're confusing me a bit because I was under the impression that you were creating the class from index2.php in your original index.php file. If you are running index.php, it has no visibility to what goes on in the index2.php unless you include it as i described above (in which case it might also spit out all that html so you get some garbled html) OR unless you store the results of index.php in a file or in a database or something like that.

Separate PHP files run independently of each other. What happens in one is of no consequence in the other unless you take pains to make it have consequence...like including it, writing data to a cookie for the other page to find, storing data in a DB for the other page to query, writing data to a file for the other page to read.

If you have code in index2.php that you want to also run in index1.php, you should move that code to a 3rd file and maybe call it 'shared.php' and then both index.php and index2.php can access it like this:

include('shared.php');

4) the way to do that last thing is to only define the instance of 'StringHandlerClass' inside the function. like this:

// nobody's heard of $my_class at this point
function my_function($arg) {
  $my_class = new StringHandlerClass();
  // use it while it's hot!
} // my_function()

// $my_class doesn't exist here either...it will only consume memory inside the function

cioannou

Thank you for taking the time to answer but now I'm even more confused,

by looking around I stumbled upon various applications and frameworks and I cannot understand how they manage to reference their global $db or $user class inside the various pages of the application.

sneakyimp

if i write a page which defines a variable, that variable will be defined in all the pages that get included by that file. OR if I define a variable in a file that gets included (like 'var_definitions.php'), that variable will be defined in the main file or all files that get included after i include var_definitions.php.

$message = "Save the environment.";

// the following files will all know what my message is
include('1.php');
include('2.php');
include('3.php');

// i can also include my definitions...the previous files won't know
// what these values are because they've already executed
// but any subsequent files (like 'functions.php') will

include('var_definitions.php');
include('functions.php');

The one other issue regarding whether a variable will be defined is SCOPE. in order for a global variable to be available inside a function, you have to tell your function that it exists outside the function using the 'global' keyword. for instance, suppose i had my_function defined in functions.php and i wanted to reference $message:

/* FUNCTIONS.PHP */

function my_function() {
  global $message

  echo $message;
}

that would output my variable just like i defined it above.

try reading this:
http://us3.php.net/global

ahundiak

cioannou wrote:
Question 1:

e.g. (index.php)
echo "Hello World!";

var $sample;

$sample = &new MyClass($value1, $value2, $value3);
What happens if I refresh the page? Is a new class created everytime I refresh?

Yes. A new class instance is created each time the page is refreshed.

Question 2:

From the previous page I need to call another page (e.g. index2.php) and
use the class created in index.php, how can I do that?

You can store the instance in a SESSION variable and then restore.

Question 3:

In index2.php I create a new class
require_once ('test_class.php');

$testvar = &new TestClass($value1);
What happens with this class when the user returns to index.php, is "TestClass" still available somehow?

The $testvar is destroyed when index2.php finished executation. It is not longer available unless again you store it in a session.

Qustion 4:
May I use "global" and "local" classes? e.g. MySQLClass is global for the whole site BUT "StringHandlerClass" is only locally (inside a function perhaps) used for a specific purpose and then must be removed from memory.

Yes you can. Use the global keyword to access a global instance.

Thing to keep in mind is that HTTP is a stateless protocol which means that each request is independent of each other request. So index2.php is not going to know anything about index.php unless you use a session of some sort.

And be carefull with your terminology. Understand the difference between a class definition and a class instance.