<?php
$fp = fopen('lock.txt', 'w');

function microtime_float()
{
      list($usec, $sec) = explode(" ", microtime());
          return ((float)$usec + (float)$sec);
}


$time_start = microtime_float();

/* Activate the LOCK_NB option on an LOCK_EX operation */
if(!flock($fp, LOCK_EX)) {
    echo 'Unable to obtain lock';
    exit(-1);
} else {
  echo "Got the lock";
}


$time_end = microtime_float();
$time = $time_end - $time_start;

echo "Got lock within  $time seconds\n";

/* ... */

fclose($fp);