PHP Dynamic Drop Down Menu

You need to write a recursive function to do this and have it call itself. I haven't tested this out, but I think it should get you started. I wouldn't endorse this function's efficiency since it runs through every item in the array and does a comparison even though you're going to only need an item or two from each run (likely).

PHP:

$arr = array(...);
function output_lis($parentID = NULL){
    global $arr;
    $stack = array(); //create a stack for our <li>'s 
    foreach($arr as $a){ 
        $str = '';
            //if the item's parent matches the parentID we're outputting...
        if($a['parent']===$parentID){ 
            $str.='<li>'.$a['title'];

                    //Pass this item's ID to the function as a parent, 
                    //and it will output the children
            $subStr = output_lis($a['id']);
            if($subStr){
                $str.='<ul>'.$subStr.'</ul>';
            }

            $str.='</li>';
            $stack[] = $str;
        }
    }
    //If we have <li>'s return a string 
    if(count($stack)>0){
        return join("\n",$stack);
    }

    //If no <li>'s in the stack, return false 
    return false;
}

Then output this on your page. Something like:

<ul>
    <?php echo output_lis(); ?>
</ul>

Here is my sample array:

$arr = array(
        array('title'=>'home','parent'=>NULL,'id'=>1), 
        array('title'=>'sub1','parent'=>1,'id'=>2), 
        array('title'=>'sub2','parent'=>1,'id'=>3), 
        array('title'=>'about us','parent'=>NULL,'id'=>4), 
        array('title'=>'sub3','parent'=>4,'id'=>5), 
        array('title'=>'sub4','parent'=>4,'id'=>6), 
    );