Add semaphore from sync extension

src/Sync/ExtSyncSemaphore.php

@@ -0,0 +1,41 @@
+<?php
+namespace Icicle\Concurrent\Sync;
+
+use Icicle\Concurrent\Exception\SemaphoreException;
+
+/**
+ * A wrapper for a semaphore based on the "sync" extension.
+ */
+class ExtSyncSemaphore implements SemaphoreInterface
+{
+    /**
+     * @var \SyncSemaphore A semaphore instance.
+     */
+    private $semaphore;
+
+    /**
+     * Creates a new semaphore object.
+     *
+     * @param int $maxLocks The maximum number of processes that can lock the semaphore.
+     */
+    public function __construct($maxLocks = 1)
+    {
+        $this->semaphore = new \SyncSemaphore(null, $maxLocks);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function lock()
+    {
+        $this->semaphore->lock(-1);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function unlock()
+    {
+        $this->semaphore->unlock();
+    }
+}