gistfile1.php

<?php
function array_map_eval($code, $array)
{
    $retval=array();
    foreach($array as $_) $retval[]=eval($code);
    return $retval;
}

$input = array();
for($i=0;$i<2;++$i)
{
    $obj = new stdclass;
    $obj->id = $i;
    $obj->foo = "bar$i";
    $input[]=$obj;
}

$output = array_map_eval('return $_->id;', $input);
print_r($input);
print_r($output);
exit;

/* OUTPUT:

Array
(
    [0] => stdClass Object
        (
            [id] => 0
            [foo] => bar0
        )

    [1] => stdClass Object
        (
            [id] => 1
            [foo] => bar1
        )

)
Array
(
    [0] => 0
    [1] => 1
)

*/
?>