Prevent concurrent execution of PHP Script

nonConcurrentScript.php

/**
 * Creates an exclusive lock
 * @param $lockDir Directory to create/access lock file
 * @param $lockName Name for lock
 * @param $lockHandle Reference to scope var to maintain file handle after lock call completes
 * @return bool
 * @throws Exception on $lockdir not writable, unable to create file handle, unable to obtain ex_lock
 */
function lock($lockDir, $lockName, &$lockHandle)
{
    $lockFileName = $lockName . '.lock';
    $lockFullPath = $lockDir . DIRECTORY_SEPARATOR . $lockFileName;

    if (!is_writable($lockDir)) {
        throw new Exception('Unable to create lock, ' . $lockDir . ' must be writable.');
    }

    $lockHandle = fopen($lockFullPath, 'w');

    if (!$lockHandle) {
        throw new Exception('Unable to write lock file: ' .$lockFullPath);
    }

    if (!flock($lockHandle, LOCK_EX | LOCK_NB)) {
        throw new Exception($lockName . ' Currently locked.');
    }

    return true;
}

/* Test lock() */

$lockDir = __DIR__;
$scriptName = basename(__FILE__, '.php');
$lockHandle = null;

try {
    lock($lockDir, $scriptName, $lockHandle);
} catch (Exception $e) {
    exit($e->getMessage());
}

echo "$scriptName Script Running...";
sleep(10);

echo "\nScript finished!";