I'm at a loss here. I've defined an abstract superclass in one file and a subclass in another. I have required the super-classes file and the stack trace reports to find an include it. However, it then returns an error when it hits the 'extends' line: Fatal error: Class 'HTMLBuilder' not found in View/Markup/HTML/HTML4.01/HTML4_01Builder.php on line 7. I had this working with another class tree that uses factories a moment ago. I just added the builder layer in between the factories and the consumer. The factory layer looked almost exactly the same in terms of includes and dependencies. So that makes me think I must have done something silly that's causes the HTMLBuilder.php file to not be included correctly or interpreted correctly or some such.

Here's the full stack trace (paths slightly altered):

#   Time    Memory  Function    Location
1   0.0001  53904   {main}( )   ../index.php:0
2   0.0002  67600   require_once( 'View/Page.php' ) ../index.php:3
3   0.0003  75444   require_once( 'View/Sections/SectionFactory.php' )  ../Page.php:4
4   0.0003  81152   require_once( 'View/Sections/HTML/HTMLSectionFactory.php' ) ../SectionFactory.php:3
5   0.0004  92108   require_once( 'View/Sections/HTML/HTMLTitlebarSection.php' )    ../HTMLSectionFactory.php:5
6   0.0005  99716   require_once( 'View/Markup/HTML/HTMLBuilder.php' )  ../HTMLTitlebarSection.php:3
7   0.0005  103580  require_once( 'View/Markup/MarkupBuilder.php' ) ../HTMLBuilder.php:3
8   0.0006  124120  require_once( 'View/Markup/HTML/HTML4.01/HTML4_01Builder.php' ) ../MarkupBuilder.php:3

Here's the code in question:

Parent class (View/Markup/HTML/HTMLBuilder.php):

<?php

require_once('View/Markup/MarkupBuilder.php');

abstract class HTMLBuilder extends MarkupBuilder {

    public abstract function getLink($text, $href);

    public abstract function getImage($src, $alt);

    public abstract function getDivision($id, array $classes=NULL, array $children=NULL);

    public abstract function getParagraph($text, array $classes=NULL, $id=NULL);

}

?>

Child Class, (View/Markup/HTML/HTML4.01/HTML4_01Builder.php):

<?php

require_once('HTML4_01Factory.php');
require_once('View/Markup/HTML/HTMLBuilder.php');


class HTML4_01Builder extends HTMLBuilder {
    private $factory;

    public function __construct() {
        $this->factory = new HTML4_01Factory(); 
    }

    public function getLink($href, $text) {
        $link = $this->factory->getA();
        $link->addAttribute('href', $href);
        $link->addChild($this->factory->getText($text));
        return $link;   
    }

    public function getImage($src, $alt) {
        $image = $this->factory->getImg();
        $image->addAttribute('src', $src);
        $image->addAttribute('alt', $alt);
        return $image;
    }

    public function getDivision($id, array $classes=NULL, array $children=NULL) {
        $div = $this->factory->getDiv();
        $div->setID($id);
        if(!empty($classes)) {
            $div->addClasses($classes);
        }   
        if(!empty($children)) {
            $div->addChildren($children);   
        }
        return $div;
    }

    public function getParagraph($text, array $classes=NULL, $id=NULL) {
        $p = $this->factory->getP();
        $p->addChild($this->factory->getText($text));
        if(!empty($classes)) {
            $p->addClasses($classes);   
        }
        if(!empty($id)) {
            $p->setID($id); 
        }
        return $p;
    }

}


?>

I would appreciate any and all ideas. I'm at a complete loss here as to what is going wrong. I'm sure it's something stupid I just can't see...