[php] How to use a PHP class from another file?

Let's say I've got two files class.php and page.php

class.php

<?php 
    class IUarts {
        function __construct() {
            $this->data = get_data('mydata');
        }
    }
?>

That's a very rudamentary example, but let's say I want to use:

$vars = new IUarts(); 
print($vars->data);

in my page.php file; how do I go about doing that? If I do include(LIB.'/class.php'); it yells at me and gives me Fatal error: Cannot redeclare class IUarts in /dir/class.php on line 4

Use include_once instead.
This error means that you have already included this file.

include_once(LIB.'/class.php');

use

require_once(__DIR__.'/_path/_of/_filename.php');

This will also help in importing files in from different folders. Try extends method to inherit the classes in that file and reuse the functions

Use include("class.classname.php");

And class should use <?php //code ?> not <? //code ?>

In this case, it appears that you've already included the file somewhere. But for class files, you should really "include" them using require_once to avoid that sort of thing; it won't include the file if it already has been. (And you should usually use require[_once], not include[_once], the difference being that require will cause a fatal error if the file doesn't exist, instead of just issuing a warning.)