<?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);

PD9waHAKCmZ1bmN0aW9uIGFycmF5X21hcF9hc3NvY2lhdGl2ZSgkY2FsbGJhY2ssICRhcnJheSl7CiAgICAkcmVzdWx0ID0gYXJyYXkoKTsKICAgIGZvcmVhY2ggKCRhcnJheSBhcyAka2V5ID0+ICR2YWx1ZSl7CiAgICAgICAgJHJlc3VsdFtdID0gY2FsbF91c2VyX2Z1bmMoJGNhbGxiYWNrLCAka2V5LCAkdmFsdWUpOwogICAgfQogICAgcmV0dXJuICRyZXN1bHQ7Cn0KCmZ1bmN0aW9uIGNhbGxiYWNrKCRrZXksICR2YWx1ZSl7CiAgICByZXR1cm4gJGtleSAuICctJyAuICR2YWx1ZTsKfQoKJGRhdGEgPSBhcnJheSgKICAgIDM1ID0+ICczJywKICAgIDI0ID0+ICc2JywKICAgIDcyID0+ICcxJywKICAgIDE2ID0+ICc1JywKICAgIDgxID0+ICcyJywKKTsKCiRyZXN1bHQgPSBpbXBsb2RlKCd8JywgYXJyYXlfbWFwX2Fzc29jaWF0aXZlKCdjYWxsYmFjaycsICRkYXRhKSk7Cgp2YXJfZHVtcCgkcmVzdWx0KTs=