diff --git a/Tests/IOTestCase.php b/Tests/IOTestCase.php
index d7ccb71..6cd2301 100644
--- a/Tests/IOTestCase.php
+++ b/Tests/IOTestCase.php
@@ -53,7 +53,7 @@ protected function executeCliCommand($command, $workingDir = null)
         ];
 
         $options = null;
-        if (stripos(PHP_OS, 'WIN') === 0) {
+        if (\stripos(\PHP_OS, 'WIN') === 0) {
             $options = ['bypass_shell' => true];
         }