Question

How can I turn the first array in to the second one? The goal is to create an array that shows the hierarchy, based on location_id and parent_id. Each location_name should be in an array of which the key is its parent_id.

Ignore the values I gave to location_name. No value for parent_id == NULL, these are the top level items.

First array:

    Array
    (
        [0] => stdClass Object
            (
                [location_id] => 1
                [location_name] => Town 1
                [parent_id] => 
            )

        [1] => stdClass Object
            (
                [location_id] => 2
                [location_name] => Town 1.1
                [parent_id] => 
            )

        [2] => stdClass Object
            (
                [location_id] => 3
                [location_name] => Town 1.2
                [parent_id] => 1
            )

        [3] => stdClass Object
            (
                [location_id] => 4
                [location_name] => Town 1.3
                [parent_id] => 1
            )

        [4] => stdClass Object
            (
                [location_id] => 5
                [location_name] => town 1.1.1
                [parent_id] => 2
            )

        [5] => stdClass Object
            (
                [location_id] => 6
                [location_name] => Town 1.1.2
                [parent_id] => 3
            )
);

Resulting array should be:

Array(
        'Town 1' = array(
            'town 1.2',
            'town 1.3' = array(
                 'town 1.1.2'
             )
        ),
        'Town 2' = array(
             'town 1.1.1'
        )
    );

EDIT: working solution based on Rijk’s answer

function _order_locs($parent, $array)
    {
        $return = array();
        foreach ( $array as $town )
        {
            if ( $town->parent_id == $parent )
            {
                $set = $this->_order_locs( $town->location_id, $array );               
                if( $this->_menu_is_parent($town->location_id, $array) ) $return[$town->location_name] = $set;
                else $return[] = $town->location_name;            
            }
        }

        return $return;
    }

    function _menu_is_parent($id, $array)
    {
        foreach( $array as $a )
        {
            if( $a->parent_id == $id ) return TRUE;
        }
    }

Answer 1

You have to loop through it, using a recursive function (one that calls itself):

function getChilds( $parent, $array ) {
    $return = array();
    foreach ( $array as $town ) {
        if ( $town['location_id'] == $parent ) {
            $return[] = array(
                'name' => $town['location_name'],
                'childs' => getChilds( $town['location_id'], $array )
            );
        }
    }
    return $return;
}

$towns_tree = getChilds( 0, $towns );

Might not work right off the bat, but that gives you a nice oppurtunity to play with the code and get familiar with this concept 😉