HQ9 interpreter

hq9.php

<?php
	define('SAFE_MODE', 42);
	
	echo '<pre>';
	
	function throw_exception($error) {
		die('--- EXCEPTION: ' . $error);
	}
	
	if (!isset($_GET['source'])) {
		$_GET['source'] = '';
	}
	
	$source = str_replace(' ', '+', $_GET['source']);
	$length = strlen($source);
	
	function s($position) {
		global $source, $length;
		
		if ($position < 0 || $position >= $length) {
			return '';
		}
		
		return strtolower($source[$position]);
	}
	
	$COUNTER = 0;
	class Superclass {}
	class Subclass extends Superclass { public function throwException() {} }
	
	for ($i = 0; $i < $length; ++$i) {
		if (s($i) == 'h') {
			echo 'Hallo World', "\n";
		}
		elseif (s($i) == 'q') {
			echo $source, "\n";
		}
		elseif (s($i) == '9') {
			$j = 100;
			while (--$j != 1) {
				echo $j, ' bottles of beer', "\n",
				'you take one down, pass it around,', "\n",
				$j-1, ' bottles of beer on the wall.', "\n\n";
			}
			echo '1 bottle of beer', "\n",
			'you take it down, pass it around,', "\n",
			'no more bottles of beer on the wall.', "\n\n";
		}
		elseif (s($i) == '+') {
			++$COUNTER;
			
			if (s($i-1) == '+') {
				new Subclass();
			}
		}
		elseif (s($i) == '-') {
			if (s($i-1) == '') {
				throw_exception('syntax error');
			}
			elseif (s($i-1) == 'h') {
				throw_exception('I/O error');
			}
			elseif (s($i-1) == 'q') {
				function infinite_recursion() {
					infinite_recursion();
				}
				if (!defined('SAFE_MODE')) {
					infinite_recursion();
				}
				else {
					throw_exception('infinite recursion detected');
				}
			}
			elseif (s($i-1) == '9') {
				if (!defined('SAFE_MODE')) {
					while (1) {};
				}
				else {
					throw_exception('infinite loop detected');
				}
			}
			elseif (s($i-1) == '+') {
				ob_start();
				1/0;
				ob_end_clean();
				throw_exception('devision by zero');
			}
			elseif (s($i-1) == '+' && s($i-2) == '+') {
				$obj = new Subclass();
				$obj->throwException();
				throw_exception('uncaught exception');
			}
		}
	}