Override's some of Parsedown's features to add support for github style task lists. You must build your own serverside & client side components for updating the markdown. The value of the checkbox is it's index of the checkbox based on all of the checkboxes found in the markdown.

TaskListParsedown.php

<?php
/**
 * Override's some of Parsedown's features to add support for github style task lists.
 * You must build your own serverside & client side components for updating the markdown.
 * The value of the checkbox is it's index of the checkbox based on all of the checkboxes found in the markdown.
 * @see https://github.com/erusev/parsedown
 */
class TaskListParsedown extends Parsedown {
    protected $taskListIndex=0;
    
    #
    # List
    protected function identifyList($Line) {
        list($name, $pattern)=($Line['text'][0]<='-' ? array('ul', '[*+-]'):array('ol', '[0-9]+[.]'));
        
        if(preg_match('/^('.$pattern.'[ ]+)(.*)/', $Line['text'], $matches)) {
            $Block=array(
                'indent'=>$Line['indent'],
                'pattern'=>$pattern,
                'element'=>array(
                    'name'=>$name,
                    'handler'=>'elements',
                ),
            );
            
            $Block['li']=$this->identifyTaskList($matches[2], $Block);
            
            $Block['element']['text'][]=&$Block['li'];
            
            return $Block;
        }
    }

    protected function addToList($Line, array $Block) {
        if($Block['indent']===$Line['indent'] && preg_match('/^'.$Block['pattern'].'[ ]+(.*)/', $Line['text'], $matches)) {
            if(isset($Block['interrupted'])) {
                $Block['li']['text'][]= '';

                unset($Block['interrupted']);
            }
            
            unset($Block['li']);
            
            $Block['li']=$this->identifyTaskList($matches[1], $Block);
            
            $Block['element']['text'][]=&$Block['li'];
            
            return $Block;
        }
        
        if(!isset($Block['interrupted'])) {
            $text=preg_replace('/^[ ]{0,4}/', '', $Line['body']);
            
            $Block['li']['text'][]=$text;
            
            return $Block;
        }
        
        if($Line['indent']>0) {
            $Block['li']['text'][]='';
            
            $text = preg_replace('/^[ ]{0,4}/', '', $Line['body']);
            
            $Block['li']['text'][]=$text;
            
            unset($Block['interrupted']);
            
            return $Block;
        }
    }
    
    protected function identifyTaskList($lineText, &$Block) {
        if(preg_match('/^(\s*)\[(\s|x)\](.*?)$/i', $lineText, $matches)) {
            $result=array();
            
            if(count($matches)>3) {
                if(!array_key_exists('taskList', $Block['element'])) {
                    if(!array_key_exists('attributes', $Block['element'])) {
                        $Block['element']['attributes']=array('class'=>'taskList');
                    }else {
                        $Block['element']['attributes']['class']='taskList';
                    }
                    
                    $Block['element']['taskList']=true;
                }
                
                
                if(!empty($matches[1])) {
                    $result[]=$matches[1];
                }
                
                $checkID='md-check-'.uniqid(time());
                $result['input']=array(
                                        'name'=>'input',
                                        'attributes'=>array(
                                                            'type'=>'checkbox',
                                                            'name'=>$checkID,
                                                            'id'=>$checkID,
                                                            'value'=>'1'
                                                        )
                                    );
                
                if(strtolower($matches[2])=='x') {
                    $result['input']['attributes']['checked']='checked';
                }
                
                $result['label']=array(
                                        'name'=>'label',
                                        'handler'=>'line',
                                        'attributes'=>array('for'=>$checkID),
                                        'text'=>$matches[3]
                                    );
                
                
                $lineText='<p>';
                foreach($result as $item) {
                    $lineText.=$this->element($item);
                }
                
                $lineText.='</p>';
                
                //Increment the index
                $this->taskListIndex++;
            }
        }
        
        
        return array(
                    'name'=>'li',
                    'handler'=>'li',
                    'text'=>array($lineText)
                );
    }
}
?>