[Tutor] Dynamic creation of class instances...

[Erik Price]

On Wednesday, May 22, 2002, at 03:03  PM, Danny Yoo wrote:

> People with PHP experience appear to take this approach, if:
>
>     
> http://www.onlamp.com/pub/a/php/2001/05/17/php_foundations.html?page=2
>     http://www.phpbuilder.com/columns/robert20000928.php3
>
> is a indication of what an experienced PHP programmer can do with
> "variable variables".  Yikes.  So that may be one source of the 
> question.

I did take advantage of variable variables (PHP's equivalent of 
exec()ing strings of code) for a bit -- the nature of HTTP sort of 
requires it for generating unique form field names:

$data = "";
for ($i = 0; $i < count($items); $i++) {
	$data .= "<input type='hidden'
				name='uniquefield" . $i
				. "' value='" . $items[$i] . "' />\n";
}

Now assuming $items = array('apple', 'orange', 'banana'), $data is equal 
to

<input type='hidden' name='uniquefield0' value='apple' />
<input type='hidden' name='uniquefield1' value='orange' />
<input type='hidden' name='uniquefield2' value='banana' />

HTTP doesn't have any means of representing an array per se, so you need 
to create unique strings to represent each element when you have no idea 
how many elements there may be in the array.

I also use exec quite often in JavaScript.  I do it so that I can modify 
an object's name based on the argument passed to a function, for 
instance:

function chooseInput(inputName)
{
	chosenObject = eval('document.formname.' + inputName + '.value');
	if (chosenObject == '')
		document.write('There is no value in the form field called '
						+ inputName + '!');
	else
		document.write('The value of the form field called '
						+ inputName + ' is ' + chosenObject);
}

The above function is stupid and doesn't even return a value, it's just 
a subroutine, but it shows how a single function can be used in multiple 
parts of a JavaScript/HTML document affecting different form names or 
document objects based on the argument to the function.

Note that there is nothing unsafe about the use of exec() or var vars in 
either the PHP example or the JavaScript example above, since no user 
input is ever evaluated (the argument to the JavaScript function would 
be coded into the page by the programmer).  And yet each is really 
needed to perform its task.


Erik

PS:  I stopped using variable variables in PHP when I gave up on hidden 
form fields for maintaining state, sessions are just way too easy and 
can store objects very well.