<?php

mysql_query("LOCK TABLES flows READ");

if (!($pid1 = pcntl_fork())) {
    // Child #1

    $result = mysql_query("whatever");

    /* ... do processing ... */

    die();
}

if (!($pid2 = pcntl_fork())) {
    // Child #2

    $result = mysql_query("whatever");

    /* ... do processing ... */

    die();
}

// Wait for threads to finish
while (!pcntl_waitpid($pid1, $status, WNOHANG) || !pcntl_waitpid($pid2, $status, WNOHANG)) {
    usleep(100);
}

mysql_query("UNLOCK TABLES");
echo "Done.\n";