array_keys($arr) and array_keys($arr, NULL) should behave differently

In Zend, the latter form only returns keys which have a NULL
(or NULL-like if $strict==false) value.  The former returns all keys.

hphp/runtime/base/type_array.cpp

@@ -777,7 +777,7 @@ Variant Array::key(CVarRef search_value, bool strict /* = false */) const {
 
 Array Array::keys(CVarRef search_value /* = null_variant */,
                   bool strict /* = false */) const {
-  if (search_value.isNull()) {
+  if (!search_value.isInitialized()) {
     ArrayInit ai(size(), ArrayInit::vectorInit);
     for (ArrayIter iter(*this); iter; ++iter) {
       ai.set(iter.first());

hphp/runtime/base/type_array.h

@@ -115,6 +115,12 @@ class Array : protected ArrayBase {
   }
   bool valueExists(CVarRef search_value, bool strict = false) const;
   Variant key(CVarRef search_value, bool strict = false) const;
+
+  /**
+   * Note that search_value == null_variant will return all keys,
+   * while search_value == init_null_variant will return only keys
+   * for pairs where the value is a PHP NULL. (or null-like if strict is false)
+   */
   Array keys(CVarRef search_value = null_variant, bool strict = false) const;
   Array values() const;
 

hphp/test/test_code_run.cpp

@@ -2180,6 +2180,13 @@ bool TestCodeRun::TestArray() {
        "test(1);"
        "var_dump('three');");
 
+  MVCR("<?php "
+       "$a = array('a' => null, 'b' => 123, 'c' => false);"
+       "var_dump(array_keys($a));"
+       "var_dump(array_keys($a, null));"
+       "var_dump(array_keys($a, null, true));"
+      );
+
   return true;
 }
 

hphp/test/test_ext_array.cpp

@@ -356,6 +356,39 @@ bool TestExtArray::test_array_keys() {
        ")\n");
   }
 
+  {
+    Array array = CREATE_MAP3(
+      "a", init_null_variant,
+      "b", 123,
+      "c", false);
+    VS(f_print_r(f_array_keys(array), true),
+      "Array\n"
+      "(\n"
+      "    [0] => a\n"
+      "    [1] => b\n"
+      "    [2] => c\n"
+      ")\n");
+    VS(f_print_r(f_array_keys(array, init_null_variant), true),
+      "Array\n"
+      "(\n"
+      "    [0] => a\n"
+      "    [1] => c\n"
+      ")\n");
+    // null_variant is uninitialized, so it should act like not passing a value
+    VS(f_print_r(f_array_keys(array, null_variant), true),
+      "Array\n"
+      "(\n"
+      "    [0] => a\n"
+      "    [1] => b\n"
+      "    [2] => c\n"
+      ")\n");
+    VS(f_print_r(f_array_keys(array, init_null_variant, true), true),
+      "Array\n"
+      "(\n"
+      "    [0] => a\n"
+      ")\n");
+  }
+
   return Count(true);
 }