<?php

$database = 'DATABASE';
$username = 'USERNAME';
$password = 'PASSWORD'

try {
    $dbh = new PDO('mysql:host=localhost;dbname='.database,  $username, $password);
} catch (PDOException $d) {
    print "Error: " . $e->getMessage() . "\n";
}

$sth = $dbh->prepare('SELECT sum(bytes) as bytes
    FROM table
    WHERE key = :interfaceid');

$bytes = null;
$sth->bindColumn('bytes', $bytes);

$interface_id = null;
$sth->bindParam(':interfaceid', $interfaceid, PDO::PARAM_INT);

$interface_ids = array(3, 5, 6);

foreach($interface_ids as $interfaceid) {
    if( exec_fetch($sth) )
        print "Got $bytes\n";
}


function exec_fetch(&$sth) {
    try {
        $sth->execute();
        return $sth->fetch();
    } catch (PDOException $e) {
        print "Error: " . $e->getMessage() . "\n";
        return false;
    }
}