<?php
function fake_provider($code)
{
    if ($code==='cat') return 'meow';
    elseif ($code==='dog') return 'woof';
    elseif ($code==='snake') return 'hiss';
}

class Req implements ArrayAccess
{
    public $codes, $completed=false, $result;
    public function __construct(...$codes)
    {
        $this->codes=$codes;
    }

    public function complete()
    {
        while (!$this->completed()) $this->progress();
    }

    public function progress()
    {
        $code=array_pop($this->codes);
        $this->result[$code]=fake_provider($code);
    }

    public function completed() { return empty($this->codes); }

    public function offsetExists($offset) { if ($this->completed()) return array_key_exists($offset, $this->result); else $this->premature_call(); }
    public function offsetGet($offset) { if ($this->completed()) return $this->result[$offset]; else $this->premature_call(); }
    public function offsetSet ( $offset , $value ) { $this->set_is_forbidden(); }
    public function offsetUnset ( $offset ) { $this->set_is_forbidden(); }
    public function premature_call() { throw new Exception('premature req call'); }
    public function set_is_forbidden() { throw new Exception('set is forbidden'); }
}

function process_values()
{
    $need=new Req('cat', 'snake');
    $tries=0;
    echo("phase once\n");
    while (!$need->completed())
    {
        $tries++;
        echo "try #$tries\n";
        yield $need;
    }
    echo("phase two\n");
    yield "cat goes $need[cat] and snake goes $need[snake]";
}

for ($gen=process_values(); $gen->valid(); $gen->next())
{
    $response=$gen->current();
    echo("response is: ".print_r($response, true)."\n");
    if ($response instanceof Req) $response->progress();
}

echo('result is: '.print_r($response, true));

/*
----
Output is:

phase once
try #1
response is: Req Object
(
    [codes] => Array
        (
            [0] => cat
            [1] => snake
        )

    [completed] =>
    [result] =>
)

try #2
response is: Req Object
(
    [codes] => Array
        (
            [0] => cat
        )

    [completed] =>
    [result] => Array
        (
            [snake] => hiss
        )

)

phase two
response is: cat goes meow and snake goes hiss
result is: cat goes meow and snake goes hiss
*/
?>