lock multi-thread execution

crontab/crawler.php

@@ -1,5 +1,13 @@
 <?php
 
+// Lock multi-thread execution
+$semaphore = sem_get(crc32('crontab.crawler'), 1);
+
+if (false === sem_acquire($semaphore, true)) {
+
+  exit;
+}
+
 // Load system dependencies
 require_once('../config/app.php');
 require_once('../library/curl.php');