<?php


$ch = curl_init();
$headerdata = array("Origin: https://www.facebook.com",
"User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.95 Safari/537.36",
"Content-Type: application/x-www-form-urlencoded",
"Referer: https://www.facebook.com/messages/123456789",
"Cookie:  your cookie data goes here");
curl_setopt($ch, CURLOPT_URL, 'https://www.facebook.com/ajax/mercury/thread_info.php');
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headerdata);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
// [limit]=20000 messages
curl_setopt($ch,CURLOPT_POSTFIELDS, "messages[thread_ids][id.123456789][offset]=0&messages[thread_ids][id.123456789][limit]=20000&&client=web_messenger&__user=123456789&__a=1&__dyn=123456789ABCDEF&__req=f&fb_dtsg=ABCDEF&ttstamp=123456789");
$x = curl_exec($ch); curl_close($ch);

$x = str_replace("for (;;);", "", $x);

$data= json_decode($x);

foreach($data->payload->actions as $value) {

	echo $value->timestamp . "\t" . $value->author . "\t" . str_replace("\n", " ", $value->body). "\n";
}

?>