diff --git a/src/Functions.php b/src/Functions.php
index 37a053a..0bc6a6f 100644
--- a/src/Functions.php
+++ b/src/Functions.php
@@ -56,6 +56,10 @@ protected function restoreFunctions()
      */
     protected function defineFunction($name, \Closure $closure)
     {
+        if (! Runkit::isAvailable()) {
+            $this->markTestSkipped('defineFunction() requires Runkit be available, skipping.');
+        }
+
         if (function_exists($name)) {
             throw new FunctionExistsException(sprintf(
                 'Function %1$s() already exists. You may redefine it using %2$s::redefineFunction() instead.',
@@ -130,6 +134,10 @@ protected function deleteFunction($name)
             return $this;
         }
 
+        if (! Runkit::isAvailable()) {
+            $this->markTestSkipped('deleteFunction() requires Runkit be available, skipping.');
+        }
+
         $namespaced = Runkit::makeNamespaced($name);
 
         if (! Runkit::function_rename($name, $namespaced)) {