<?php

function array_map_associative($callback, $array){
    $result = array();
    foreach ($array as $key => $value){
        $result[] = call_user_func($callback, $key, $value);
    }
    return $result;
}

function callback($key, $value){
    return $key . '-' . $value;
}

$data = array(
    35 => '3',
    24 => '6',
    72 => '1',
    16 => '5',
    81 => '2',
);

$result = implode('|', array_map_associative('callback', $data));

var_dump($result);