I am attempting to convert a wordpress plugin I authored procedurally to become class based, and it is no longer working for many unknown reason. And So I authored cut lower my wordpress plugin code to become about as minimilistic as you possibly can. If this runs, it returns without any content.

    <?PHP
/**
 * Plugin Name:   A1 Test 
 * Version:       1.0.0
 * Plugin URI:    
 * Description:   Test
 * Author:        
 * Author URI:     
 */
if ( ! class_exists("TestingDemo") )
{
    class TestingDemo
    {
        function TestingDemo_filter( $buffer )
        {
            $buffer = preg_replace_callback( '@\[testing\s*=\s*"(.*?)\s*"\]@si',
                array(&$this, "TestingDemo_replace"), $buffer );
        }
        function TestingDemo_replace( $matches )
        {
            $message = $matches[1];
            return "Testing Worked.....   {$message}";
        }
    }
}
if ( class_exists("TestingDemo") )
{
    $TestingDemos = new TestingDemo();
}
if ( isset($TestingDemos) )
{
    add_filter( 'the_content', array(&$TestingDemos, 'TestingDemo_filter') );
}

In my opinion you'll need a return value for the TestingDemo_filter() function. A Wordpress filter function must have a string as input, and return the modified string. Since you've setup TestingDemo_filter() because the actual filter function, it is going to require a return value.

Edit

I simply examined your code, also it certainly works whenever you give a return statement to TestingDemo_filter().