Attempting various singleton base class methods described on this page, I have created a base class and bridge function that allows it to work without get_called_class() if it's not available.  Unlike other methods listed here, I chose not to prevent use of __construct() or __clone().

<?php
abstract class Singleton {
    protected static $m_pInstance;
    final public static function getInstance(){
        $class = static::getClass();
        if(!isset(static::$m_pInstance[$class])) {
            static::$m_pInstance[$class] = new $class;
        }
        return static::$m_pInstance[$class];
    }
    final public static function getClass(){
        return get_called_class();
    }
}

// I don't remember where I found this, but this is to allow php < 5.3 to use this method.
if (!function_exists('get_called_class')) {
    function get_called_class($bt = false, $l = 1) {
        if (!$bt)
            $bt = debug_backtrace();
        if (!isset($bt[$l]))
            throw new Exception("Cannot find called class -> stack level too deep.");
        if (!isset($bt[$l]['type'])) {
            throw new Exception('type not set');
        }
        else
            switch ($bt[$l]['type']) {
                case '::':
                    $lines = file($bt[$l]['file']);
                    $i = 0;
                    $callerLine = '';
                    do {
                        $i++;
                        $callerLine = $lines[$bt[$l]['line'] - $i] . $callerLine;
                    } while (stripos($callerLine, $bt[$l]['function']) === false);
                    preg_match('/([a-zA-Z0-9\_]+)::' . $bt[$l]['function'] . '/', $callerLine, $matches);
                    if (!isset($matches[1])) {
                        // must be an edge case.
                        throw new Exception("Could not find caller class: originating method call is obscured.");
                    }
                    switch ($matches[1]) {
                        case 'self':
                        case 'parent':
                            return get_called_class($bt, $l + 1);
                        default:
                            return $matches[1];
                    }
                // won't get here.
                case '->': switch ($bt[$l]['function']) {
                        case '__get':
                            // edge case -> get class of calling object
                            if (!is_object($bt[$l]['object']))
                                throw new Exception("Edge case fail. __get called on non object.");
                            return get_class($bt[$l]['object']);
                        default: return $bt[$l]['class'];
                    }

                default: throw new Exception("Unknown backtrace method type");
            }
    }
}

class B extends Singleton {
}

class C extends Singleton {
}

$b = B::getInstance();
echo 'class: '.get_class($b);

echo '<br />';

$c = C::getInstance();
echo echo 'class: '.get_class($c);
?>
This returns:
class: b
class: c