PHP PDO returning inconsistent results for SELECT

I have an issue with PHP/PDO and MySQL

When I run a query with SQL_CALC_FOUND_ROWS and then select FOUND_ROWS(), 80% of the time its returning 0, and the rest of the time FOUND_ROWS is accurate

I've reduced it to a simple test loop, but this works fine on my dev server, but in production the test case is having the inconsistency problem.

Running the same queries from MySQL command line works correctly even in production, so it appears to be a PHP/PDO problem

PHP 5.5.28 - using mysqlnd 5.0.11-dev Percona Server 5.6.25-73.1-log on CentOS 6.6 (Final)

Can anyone help please? I've tried everything I can think of, and I'm tearing my hair out

<?php
require_once "../consts.php";

$nolimit_query = "select SQL_CALC_FOUND_ROWS targetusers.u_id
  FROM 
    users targetusers 
    LEFT JOIN user_extra targetuserextra ON (targetuserextra.ue_userid = targetusers.u_id) 
    LEFT JOIN countries ON (c_id = targetusers.u_country) 
    LEFT JOIN cities ON (cities.ct_countryid = c_id and cities.ct_id = targetusers.u_city) 
    LEFT JOIN userimages ON (targetusers.u_primaryimage = userimages.ui_id and userimages.ui_userid = targetusers.u_id and userimages.ui_imagetype = 'P') 
  WHERE 
    (targetusers.u_deleted = 0) and 
    (targetusers.u_id NOT IN (19, 32, 115)) and 
    (targetusers.u_active = 1) and 
    (targetusers.u_confirmed=5) order by u_lastupdated DESC LIMIT 10, 10";

// already tried this soln suggested elsewhere -- defaults to off for my PHP anyway though
ini_set("mysql.trace_mode", 0);
ini_set("display_errors", 1);

// set up PDO connection
$dbh = new PDO(
    "mysql:host=" . Config::Get()->DB_SERVER . ";dbname=" . Config::Get()->DATABASE_NAME . ";charset=utf8", 
    Config::Get()->DB_USERNAME, 
    Config::Get()->DB_PASSWORD
);
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$dbh->setAttribute(PDO::ATTR_PERSISTENT, false);

echo $dbh->getAttribute(PDO::ATTR_CLIENT_VERSION) . "<br>";
echo $dbh->getAttribute(PDO::ATTR_DRIVER_NAME) . "<br>";
echo $dbh->getAttribute(PDO::ATTR_SERVER_INFO) . "<br>";
echo $dbh->getAttribute(PDO::ATTR_SERVER_VERSION) . "<br><br>";

echo date("r") . "<br>";
for ($i=0; $i<5; $i++)
{
    // run query above
    $stmt = $dbh->prepare($nolimit_query);
    if ($stmt === FALSE)
    {
        die($stmt->errorInfo());
    }

    // get result set 
    if (($result = $stmt->execute()) === TRUE)
    {
        $a = $stmt->fetchAll(PDO::FETCH_ASSOC);
        $stmt->closeCursor();
        $stmt = NULL;

        // this query returns inconsistent results (80% of the time it returns 0!)
        $b = $dbh->query("SELECT FOUND_ROWS() as cnt")->fetch(PDO::FETCH_ASSOC);
        echo "FOUND ROWS = " . $b["cnt"] . "<br>";      
    }
}

The output of this script is below.

Note that only 2 of the 5 loops are returning the correct value for FOUND_ROWS, with the other 3 coming back as 0

mysqlnd 5.0.11-dev - 20120503 - $Id: 15d5c781cfcad91193dceae1d2cdd127674ddb3e $
mysql
Uptime: 1857223 Threads: 1 Questions: 4446069 Slow queries: 32 Opens: 465 Flush tables: 1 Open tables: 403 Queries per second avg: 2.393
5.6.25-73.1

Fri, 04 Sep 2015 12:01:10 +0100
FOUND ROWS = 0
FOUND ROWS = 0
FOUND ROWS = 59836
FOUND ROWS = 0
FOUND ROWS = 59836

Problem solved. It turns out the NewRelic Application Monitoring Daemon or extension was interfering with the results of FOUND_ROWS()

That's SO bad :(

Disabling the extension and FOUND_ROWS is bulletproof again