diff --git a/src/collections.c b/src/collections.c
index 75e613b..5d64896 100644
--- a/src/collections.c
+++ b/src/collections.c
@@ -8,12 +8,11 @@
 #include "config.h"
 #endif
 
-#include <php.h>
+#include "php_collections.h"
+
 #include <zend_interfaces.h>
 #include <ext/standard/info.h>
 
-#include "php_collections.h"
-
 zend_object_handlers collection_handlers;
 
 zend_class_entry* collections_collection_ce;
diff --git a/src/collections_me.c b/src/collections_me.c
index dedd281..f7f5233 100644
--- a/src/collections_me.c
+++ b/src/collections_me.c
@@ -4,8 +4,6 @@
 // @Author CismonX
 //
 
-#include <php.h>
-
 #include "php_collections_me.h"
 
 ZEND_BEGIN_ARG_INFO(action_arginfo, 0)
@@ -189,6 +187,7 @@ const zend_function_entry collection_methods[] = {
     PHP_ME(Collection, removeAll, other_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, removeWhile, predicate_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, retainAll, other_arginfo, ZEND_ACC_PUBLIC)
+    PHP_ME(Collection, retainWhile, predicate_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, reverse, NULL, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, reversed, NULL, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, shuffle, NULL, ZEND_ACC_PUBLIC)
diff --git a/src/collections_methods.c b/src/collections_methods.c
index 6c2bc94..de74290 100644
--- a/src/collections_methods.c
+++ b/src/collections_methods.c
@@ -4,13 +4,12 @@
 // @Author CismonX
 //
 
-#include <php.h>
-#include <ext/standard/php_string.h>
-#include <ext/standard/php_mt_rand.h>
-
 #include "php_collections.h"
 #include "php_collections_me.h"
 
+#include <ext/standard/php_string.h>
+#include <ext/standard/php_mt_rand.h>
+
 #if PHP_VERSION_ID < 70300
 #define GC_ADDREF(p)                ++GC_REFCOUNT(p)
 #define GC_DELREF(p)                --GC_REFCOUNT(p)
@@ -742,7 +741,7 @@ PHP_METHOD(Collection, average)
             sum += Z_DVAL_P(val);
         } else {
             ERR_NOT_NUMERIC();
-            RETURN_FALSE;
+            RETURN_NULL();
         }
     ZEND_HASH_FOREACH_END();
     RETVAL_DOUBLE(sum / zend_hash_num_elements(current));
@@ -2273,6 +2272,26 @@ PHP_METHOD(Collection, retainAll)
     
 }
 
+PHP_METHOD(Collection, retainWhile)
+{
+    zend_fcall_info fci;
+    zend_fcall_info_cache fcc;
+    ZEND_PARSE_PARAMETERS_START(0, 1)
+        Z_PARAM_OPTIONAL
+        Z_PARAM_FUNC(fci, fcc)
+    ZEND_PARSE_PARAMETERS_END();
+    zend_array* current = COLLECTION_FETCH_CURRENT();
+    SEPARATE_CURRENT_COLLECTION(current);
+    INIT_FCI(&fci, 2);
+    ZEND_HASH_FOREACH_BUCKET(current, Bucket* bucket)
+        CALLBACK_KEYVAL_INVOKE(params, bucket);
+        if (!zend_is_true(&retval)) {
+            zend_hash_del_bucket(current, bucket);
+        }
+        zval_ptr_dtor(&retval);
+    ZEND_HASH_FOREACH_END();
+}
+
 PHP_METHOD(Collection, reverse)
 {
     zend_array* current = COLLECTION_FETCH_CURRENT();
@@ -2726,7 +2745,7 @@ PHP_METHOD(Collection, sum)
                 ZVAL_DOUBLE(&sum, 0.0);
             } else {
                 ERR_NOT_NUMERIC();
-                RETURN_FALSE;
+                RETURN_NULL();
             }
         }
         if (Z_TYPE_P(val) == IS_LONG) {
@@ -2735,7 +2754,7 @@ PHP_METHOD(Collection, sum)
             Z_DVAL(sum) += Z_DVAL_P(val);
         } else {
             ERR_NOT_NUMERIC();
-            RETURN_FALSE;
+            RETURN_NULL();
         }
     ZEND_HASH_FOREACH_END();
     RETVAL_ZVAL(&sum, 0, 0);
@@ -2762,7 +2781,7 @@ PHP_METHOD(Collection, sumBy)
             } else {
                 ERR_NOT_NUMERIC();
                 zval_ptr_dtor(&retval);
-                RETURN_FALSE;
+                RETURN_NULL();
             }
         }
         if (Z_TYPE(retval) == IS_LONG) {
@@ -2772,7 +2791,7 @@ PHP_METHOD(Collection, sumBy)
         } else {
             ERR_NOT_NUMERIC();
             zval_ptr_dtor(&retval);
-            RETURN_FALSE;
+            RETURN_NULL();
         }
     ZEND_HASH_FOREACH_END();
     RETVAL_ZVAL(&sum, 0, 0);
diff --git a/src/php_collections.h b/src/php_collections.h
index 34ee85a..78ab8a6 100644
--- a/src/php_collections.h
+++ b/src/php_collections.h
@@ -7,6 +7,8 @@
 #ifndef PHP_COLLECTIONS_H
 #define PHP_COLLECTIONS_H
 
+#include <php.h>
+
 extern zend_module_entry collections_module_entry;
 #define phpext_collections_ptr          &collections_module_entry
 
diff --git a/src/php_collections_me.h b/src/php_collections_me.h
index f5bab68..1399c9a 100644
--- a/src/php_collections_me.h
+++ b/src/php_collections_me.h
@@ -7,6 +7,8 @@
 #ifndef PHP_COLLECTIONS_ME_H
 #define PHP_COLLECTIONS_ME_H
 
+#include <php.h>
+
 PHP_METHOD(Collection, __construct);
 PHP_METHOD(Collection, addAll);
 PHP_METHOD(Collection, all);
@@ -77,6 +79,7 @@ PHP_METHOD(Collection, remove);
 PHP_METHOD(Collection, removeAll);
 PHP_METHOD(Collection, removeWhile);
 PHP_METHOD(Collection, retainAll);
+PHP_METHOD(Collection, retainWhile);
 PHP_METHOD(Collection, reverse);
 PHP_METHOD(Collection, reversed);
 PHP_METHOD(Collection, shuffle);
diff --git a/stubs/Collection.php b/stubs/Collection.php
index 0b33e21..895486c 100644
--- a/stubs/Collection.php
+++ b/stubs/Collection.php
@@ -88,7 +88,7 @@ class Collection implements ArrayAccess, Countable
     /**
      * Returns an average value of elements in the collection.
      *
-     * @return double|false
+     * @return double|null
      */
     function average() {}
 
@@ -664,6 +664,14 @@ class Collection implements ArrayAccess, Countable
      */
     function retainAll($elements) {}
 
+    /**
+     * Retains all elements from this collection that match the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return void
+     */
+    function retainWhile($predicate) {}
+
     /**
      * Reverses elements in the collection in-place.
      *
@@ -812,7 +820,7 @@ class Collection implements ArrayAccess, Countable
      * 
      * All elements should be of the same type, int or double. Otherwise result is undefined.
      * 
-     * @return int|double|false
+     * @return int|double|null
      */
     function sum() {}
 
@@ -824,7 +832,7 @@ class Collection implements ArrayAccess, Countable
      * Otherwise result is undefined.
      * 
      * @param callable $selector ($value, $key) -> int|double
-     * @return int|double|false
+     * @return int|double|null
      */
     function sumBy($selector) {}
 
diff --git a/tests/029-remove-while.phpt b/tests/029-remove-retain-while.phpt
similarity index 67%
rename from tests/029-remove-while.phpt
rename to tests/029-remove-retain-while.phpt
index 5593776..733e81c 100644
--- a/tests/029-remove-while.phpt
+++ b/tests/029-remove-retain-while.phpt
@@ -6,10 +6,17 @@ $array = ['a' => 4, 'b' => 1, 'c' => 9, 'd' => -2, 'e' => 0];
 $pred_is_odd = function ($value) {
     return $value % 2;
 };
+
 $collection = Collection::init($array);
 $collection->removeWhile($pred_is_odd);
 if ($collection->toArray() != ['a' => 4, 'd' => -2, 'e' => 0]) {
     echo 'Collection::removeWhile() failed.', PHP_EOL;
 }
+
+$collection = Collection::init($array);
+$collection->retainWhile($pred_is_odd);
+if ($collection->toArray() != ['b' => 1, 'c' => 9]) {
+    echo 'Collection::retainWhile() failed.', PHP_EOL;
+}
 ?>
 --EXPECT--