<?php

$old_data = "P234324, 2011-03-23 12:34:37 \nP234434, 2011-03-23 13:34:36 \nP438794, 2011-03-23 14:34:36 \nP238924, 2011-03-23 15:34:36 \n";
$data = array();

$pairs = explode("\n", rtrim($old_data, "\n"));
foreach($pairs as $pair) {
   $components = explode(',', $pair);
   $data[$components[0]] = trim($components[1]);
}

print_r($data);

Array
(
    [P234324] => 2011-03-23 12:34:37
    [P234434] => 2011-03-23 13:34:36
    [P438794] => 2011-03-23 14:34:36
    [P238924] => 2011-03-23 15:34:36
)