Update benchmark/Benchmark.php

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: danielebarbaro

benchmark/Benchmark.php

@@ -377,7 +377,9 @@ private function benchmarkPersistence(array $documents, int $n): array
         $this->log("  Benchmarking save/open...\n");
 
         $tmpDir = sys_get_temp_dir() . '/phpvbench_' . uniqid('', true);
-        mkdir($tmpDir, 0755, true);
+        if (!is_dir($tmpDir) && !mkdir($tmpDir, 0755, true) && !is_dir($tmpDir)) {
+            throw new \RuntimeException(sprintf('Failed to create temporary benchmark directory "%s".', $tmpDir));
+        }
 
         // Save
         $saveMeasurement = $this->measure(function () use ($documents, $tmpDir): void {