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!! PHP Workflow Classes

This is a very simple approach to a general workflow framework in PHP.
''It may not even work'', but it should be worth a try.

[@
class SimpleWorkflow {
private $transitions = array();
private $initialStep = '';

public function addStep($name) {
if (array_key_exists($name, $this->transitions))
throw new Exception("Step '$name' already present.");
$this->transitions[$name] = array();
}

public function addTransition($from, $to) {
if (!array_key_exists($from, $this->transitions))
throw new Exception("Step '$from' does not exist.");
if (!array_key_exists($to, $this->transitions))
throw new Exception("Step '$to' does not exist.");
$this->transitions[$from][] = $to;
}

public function transitionExists($from, $to) {
if (!array_key_exists($from, $this->transitions)) return false;
if (!array_key_exists($to, $this->transitions)) return false;
return (in_array($to, $this->transitions[$from]));
}

public function getInitialStep() {
return $this->initialStep;
}

public function setInitialStep($name) {
if (!array_key_exists($name, $this->transitions))
throw new Exception("Step '$name' does not exist.");
$this->initialStep = $name;
}
}

class SimpleWorkflowExecutor {
private $workflow;
private $currentStep = '';
private $nextStep = '';
private $valid = false;

public function __construct(SimpleWorkflow $workflow, $currentStep, $nextStep) {
$this->workflow = $workflow;
$this->currentStep = $currentStep;
$this->nextStep = $nextStep;
$this->checkNextStep();
}

private function checkNextStep() {
$this->valid = $this->workflow->transitionExists($this->currentStep, $this->nextStep);
}

public function getNextStep() {
if (!$this->valid)
return $this->workflow->getInitialStep();
return $this->nextStep;
}
}
@]

!!! Usage

[@
function initShoppingCart() {
$wf = new SimpleWorkflow();
$wf->addStep('Select product');
$wf->addStep('Show cart');
$wf->addStep('Enter billing address');
$wf->addStep('Select shipping');
$wf->addStep('Confirm order');
$wf->addStep('Process order');

$wf->addTransition('Select product', 'Show cart');
$wf->addTransition('Show cart', 'Select product');
$wf->addTransition('Show cart', 'Enter billing address');
$wf->addTransition('Enter billing address', 'Show cart');
$wf->addTransition('Enter billing address', 'Select shipping');
$wf->addTransition('Select shipping', 'Enter billing address');
$wf->addTransition('Select shipping', 'Confirm order');
$wf->addTransition('Confirm order', 'Select shipping');
$wf->addTransition('Confirm order', 'Process order');

$wf->setInitialStep('Select product');

return $wf;
}

$shoppingCart = initShoppingcart();
$executor = new SimpleWorkflowExecutor($shoppingCart, $_POST['STEP'], $_POST['NEXT_STEP']);
$stepToExecute = $executor->getNextStep();
@]