diff --git a/run-tests.php b/run-tests.php
index 116987071ada4..5d2261434d688 100755
--- a/run-tests.php
+++ b/run-tests.php
@@ -243,7 +243,7 @@ function main(): void
         $environment["SystemRoot"] = getenv("SystemRoot");
     }
 
-    $php = null;
+    $php = '';
     $php_cgi = null;
     $phpdbg = null;