<?php

$str = "apple pear banana orange";
$str_splode = explode(' ',$str);
print_r(showCombo(array(), $str_splode));

function showCombo($str_arr, $arr){
    $ret = array();
    foreach($arr as $val){
       if(!in_array($val, $str_arr)){
           $temp = $str_arr;
           $temp[] = $val;
           $ret[$val] = showCombo($temp, $arr);
       }
    }
    return $ret;
}

?>

Array
(
    [apple] => Array
        (
            [pear] => Array
                (
                    [banana] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                )

            [banana] => Array
                (
                    [pear] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                )

            [orange] => Array
                (
                    [pear] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                    [banana] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                )

        )

    [pear] => Array
        (
            [apple] => Array
                (
                    [banana] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                )

            [banana] => Array
                (
                    [apple] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

            [orange] => Array
                (
                    [apple] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                    [banana] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

        )

    [banana] => Array
        (
            [apple] => Array
                (
                    [pear] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                )

            [pear] => Array
                (
                    [apple] => Array
                        (
                            [orange] => Array
                                (
                                )

                        )

                    [orange] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

            [orange] => Array
                (
                    [apple] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                    [pear] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

        )

    [orange] => Array
        (
            [apple] => Array
                (
                    [pear] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                    [banana] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                )

            [pear] => Array
                (
                    [apple] => Array
                        (
                            [banana] => Array
                                (
                                )

                        )

                    [banana] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

            [banana] => Array
                (
                    [apple] => Array
                        (
                            [pear] => Array
                                (
                                )

                        )

                    [pear] => Array
                        (
                            [apple] => Array
                                (
                                )

                        )

                )

        )

)