<?

$tree = <<<QQ
Gardening Tools
    Pruners & Loppers
        Loppers
        Pruners
    Shovels & Cultivators
        Cultivators & Tillers
        Manual Weeders
        Shovels
        Trowels & Spades
            Size 1
            Size 2
    More tools
    Even more tools
Other Things
More things
Gardening & Horticulture Books
    Flower Books
        Annuals
        Perennials
QQ;

$tree_struct = tree( $tree );
//var_dump( $tree_struct );

print_tree( $tree_struct );

function tree( $arg_tree ) {

    $twig = Array();
    $twig[0] = Array( 'value' => "ROOT", 'children' => Array() );

    foreach( preg_split( '/\n/', $arg_tree ) as $line ){
        $lead_regex = '/^' . str_repeat( '\s{4}', count( $twig ) - 1 ) . '/';

        while ( !preg_match( $lead_regex, $line ) ){
            array_pop($twig);
            $lead_regex = '/^' . str_repeat( '\s{4}', count( $twig ) - 1 ) . '/';
        }

        array_push($twig[count($twig)-1]['children'], Array( 'value' => preg_replace( '/^\s*/', '', $line ), 'children' => Array() ));

        #reference to the last element we created, pushed onto working twig
        array_push($twig, &$twig[count($twig)-1]['children'][count($twig[count($twig)-1]['children'])-1]);
    }

    return $twig[0];
}

function print_tree($arg_tree, $arg_depth=0) {
    print $arg_depth . str_repeat( '   ',$arg_depth) . $arg_tree['value'] . "\n";
    foreach( $arg_tree['children'] as $child ) {
        print_tree($child,$arg_depth+1);
    }
}
?>