diff --git a/.github/actions/configure-x32/action.yml b/.github/actions/configure-x32/action.yml
index a5c5df4f7971d..aa43ed2c1e7bb 100644
--- a/.github/actions/configure-x32/action.yml
+++ b/.github/actions/configure-x32/action.yml
@@ -12,7 +12,7 @@ runs:
 
         export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:/usr/lib/i386-linux-gnu/pkgconfig"
         ./buildconf --force
-        export CFLAGS="-m32 -msse2"
+        export CFLAGS="-m32 -msse2 -DGD_TEST_HELPERS"
         export CXXFLAGS="-m32 -msse2"
         export LDFLAGS=-L/usr/lib/i386-linux-gnu
         ./configure ${{ inputs.configurationParameters }} \
diff --git a/.github/scripts/windows/build_task.bat b/.github/scripts/windows/build_task.bat
index fd9a956bd38fb..f499b50fb2141 100644
--- a/.github/scripts/windows/build_task.bat
+++ b/.github/scripts/windows/build_task.bat
@@ -32,7 +32,7 @@ if "%THREAD_SAFE%" equ "0" set ADD_CONF=%ADD_CONF% --disable-zts
 if "%INTRINSICS%" neq "" set ADD_CONF=%ADD_CONF% --enable-native-intrinsics=%INTRINSICS%
 if "%ASAN%" equ "1" set ADD_CONF=%ADD_CONF% --enable-sanitizer --enable-debug-pack
 
-set CFLAGS=/W1 /WX /w14013
+set CFLAGS=/W1 /WX /w14013 /DGD_TEST_HELPERS
 
 cmd /c configure.bat ^
 	--enable-snapshot-build ^
diff --git a/ext/gd/gd.c b/ext/gd/gd.c
index 17bda3d65e2dc..f64fd465481cc 100644
--- a/ext/gd/gd.c
+++ b/ext/gd/gd.c
@@ -4326,6 +4326,73 @@ PHP_FUNCTION(imageresolution)
 }
 /* }}} */
 
+#ifdef GD_TEST_HELPERS
+static PHP_FUNCTION(imagechangedpixels)
+{
+	zval *IM1, *IM2;
+	gdImagePtr im1, im2;
+	int i, j, result = 0;
+
+	ZEND_PARSE_PARAMETERS_START(2, 2)
+		Z_PARAM_OBJECT_OF_CLASS(IM1, gd_image_ce)
+		Z_PARAM_OBJECT_OF_CLASS(IM2, gd_image_ce)
+	ZEND_PARSE_PARAMETERS_END();
+
+	im1 = php_gd_libgdimageptr_from_zval_p(IM1);
+	im2 = php_gd_libgdimageptr_from_zval_p(IM2);
+
+	ZEND_ASSERT(gdImageTrueColor(im1) && gdImageTrueColor(im2));
+	ZEND_ASSERT(gdImageSX(im1) == gdImageSX(im2) && gdImageSY(im1) == gdImageSY(im2));
+
+	for (j = gdImageSY(im1) - 1; j >= 0; --j) {
+		for (i = gdImageSX(im1) - 1; i >= 0; --i) {
+			if (gdImageTrueColorPixel(im1, i, j) != gdImageTrueColorPixel(im2, i, j)) {
+				result++;
+			}
+		}
+	}
+
+	RETURN_LONG(result);
+}
+
+static double calc_pixel_distance(gdImagePtr im1, gdImagePtr im2, int x, int y)
+{
+	int c1 = gdImageGetPixel(im1, x, y);
+	int c2 = gdImageGetPixel(im2, x, y);
+# define SQR(a) ((a) * (a))
+	return sqrt(
+		SQR(gdImageRed(im1, c1) - gdImageRed(im2, c2)) +
+		SQR(gdImageGreen(im1, c1) - gdImageGreen(im2, c2)) +
+		SQR(gdImageBlue(im1, c1) - gdImageBlue(im2, c2)));
+# undef SQR
+}
+
+static PHP_FUNCTION(calc_image_dissimilarity)
+{
+	zval *IM1, *IM2;
+	gdImagePtr im1, im2;
+	int i, j;
+	double result = 0;
+
+	ZEND_PARSE_PARAMETERS_START(2, 2)
+		Z_PARAM_OBJECT_OF_CLASS(IM1, gd_image_ce)
+		Z_PARAM_OBJECT_OF_CLASS(IM2, gd_image_ce)
+	ZEND_PARSE_PARAMETERS_END();
+
+	im1 = php_gd_libgdimageptr_from_zval_p(IM1);
+	im2 = php_gd_libgdimageptr_from_zval_p(IM2);
+
+	ZEND_ASSERT(gdImageSX(im1) == gdImageSX(im2) && gdImageSY(im1) == gdImageSY(im2));
+
+	for (j = gdImageSY(im2) - 1; j >= 0; --j) {
+		for (i = gdImageSX(im1) - 1; i >= 0; --i) {
+			result += calc_pixel_distance(im1, im2, i, j);
+		}
+	}
+
+	RETURN_DOUBLE(result);
+}
+#endif
 
 /*********************************************************
  *
diff --git a/ext/gd/gd.stub.php b/ext/gd/gd.stub.php
index 347e43e728b87..e387b19f12f18 100644
--- a/ext/gd/gd.stub.php
+++ b/ext/gd/gd.stub.php
@@ -794,3 +794,9 @@ function imagesetinterpolation(GdImage $image, int $method = IMG_BILINEAR_FIXED)
  * @refcount 1
  */
 function imageresolution(GdImage $image, ?int $resolution_x = null, ?int $resolution_y = null): array|bool {}
+
+#ifdef GD_TEST_HELPERS
+function imagechangedpixels(GdImage $im1, GdImage $im2): int {}
+
+function calc_image_dissimilarity(GdImage $im1, GdImage $im2): float {}
+#endif
diff --git a/ext/gd/gd_arginfo.h b/ext/gd/gd_arginfo.h
index 02f57e52ba940..061ed64f43e1b 100644
--- a/ext/gd/gd_arginfo.h
+++ b/ext/gd/gd_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 0f8a22bff1d123313f37da400500e573baace837 */
+ * Stub hash: d80b05c9e734f5690e5cf302dc190cef7d9ce702 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_gd_info, 0, 0, IS_ARRAY, 0)
 ZEND_END_ARG_INFO()
@@ -567,6 +567,18 @@ ZEND_BEGIN_ARG_WITH_RETURN_TYPE_MASK_EX(arginfo_imageresolution, 0, 1, MAY_BE_AR
 	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, resolution_y, IS_LONG, 1, "null")
 ZEND_END_ARG_INFO()
 
+#if defined(GD_TEST_HELPERS)
+ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_imagechangedpixels, 0, 2, IS_LONG, 0)
+	ZEND_ARG_OBJ_INFO(0, im1, GdImage, 0)
+	ZEND_ARG_OBJ_INFO(0, im2, GdImage, 0)
+ZEND_END_ARG_INFO()
+
+ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_calc_image_dissimilarity, 0, 2, IS_DOUBLE, 0)
+	ZEND_ARG_OBJ_INFO(0, im1, GdImage, 0)
+	ZEND_ARG_OBJ_INFO(0, im2, GdImage, 0)
+ZEND_END_ARG_INFO()
+#endif
+
 ZEND_FUNCTION(gd_info);
 ZEND_FUNCTION(imageloadfont);
 ZEND_FUNCTION(imagesetstyle);
@@ -701,6 +713,10 @@ ZEND_FUNCTION(imageaffinematrixconcat);
 ZEND_FUNCTION(imagegetinterpolation);
 ZEND_FUNCTION(imagesetinterpolation);
 ZEND_FUNCTION(imageresolution);
+#if defined(GD_TEST_HELPERS)
+ZEND_FUNCTION(imagechangedpixels);
+ZEND_FUNCTION(calc_image_dissimilarity);
+#endif
 
 static const zend_function_entry ext_functions[] = {
 	ZEND_FE(gd_info, arginfo_gd_info)
@@ -839,6 +855,10 @@ static const zend_function_entry ext_functions[] = {
 	ZEND_FE(imagegetinterpolation, arginfo_imagegetinterpolation)
 	ZEND_FE(imagesetinterpolation, arginfo_imagesetinterpolation)
 	ZEND_FE(imageresolution, arginfo_imageresolution)
+#if defined(GD_TEST_HELPERS)
+	ZEND_FE(imagechangedpixels, arginfo_imagechangedpixels)
+	ZEND_FE(calc_image_dissimilarity, arginfo_calc_image_dissimilarity)
+#endif
 	ZEND_FE_END
 };
 
diff --git a/ext/gd/tests/func.inc b/ext/gd/tests/func.inc
index 0f10aa7d83dee..cb62adf148578 100644
--- a/ext/gd/tests/func.inc
+++ b/ext/gd/tests/func.inc
@@ -59,6 +59,23 @@ function get_libxpm_version()
     return $version;
 }
 
+if (!function_exists("imagechangedpixels")) {
+    function imagechangedpixels(GdImage $im1, GdImage $im2): int
+    {
+        $pixels_changed = 0;
+        for ($y = imagesy($im1) - 1; $y >= 0; --$y) {
+            for ($x = imagesx($im1) - 1; $x >= 0; --$x) {
+                $c1 = imagecolorat($im1, $x, $y);
+                $c2 = imagecolorat($im2, $x, $y);
+                if ($c1 != $c2) {
+                    $pixels_changed++;
+                }
+            }
+        }
+        return $pixels_changed;
+    }
+}
+
 /**
  * Tests that an in-memory image equals a PNG file.
  *
@@ -106,16 +123,7 @@ function test_image_equals_image(GdImage $expected, GdImage $actual, bool $save_
         }
         return;
     }
-    $pixels_changed = 0;
-    for ($y = 0; $y < $exp_y; $y++) {
-        for ($x = 0; $x < $exp_x; $x ++) {
-            $exp_c = imagecolorat($expected, $x, $y);
-            $act_c = imagecolorat($actual, $x, $y);
-            if ($exp_c != $act_c) {
-                $pixels_changed++;
-            }
-        }
-    }
+    $pixels_changed = imagechangedpixels($expected, $actual);
     if (!$pixels_changed) {
         echo "The images are equal.\n";
     } else {
diff --git a/ext/gd/tests/similarity.inc b/ext/gd/tests/similarity.inc
index cb0dba77f2064..41fc5629a5d89 100644
--- a/ext/gd/tests/similarity.inc
+++ b/ext/gd/tests/similarity.inc
@@ -23,42 +23,36 @@ function get_rgb($color, &$red, &$green, &$blue)
     $blue = $color & 0xFF;
 }
 
-/**
- * Calculates the euclidean distance of two RGB values.
- *
- * @param int $color1
- * @param int $color2
- *
- * @return int
- */
-function calc_pixel_distance($color1, $color2)
-{
-    get_rgb($color1, $red1, $green1, $blue1);
-    get_rgb($color2, $red2, $green2, $blue2);
-    return sqrt(
-        pow($red1 - $red2, 2) + pow($green1 - $green2, 2) + pow($blue1 - $blue2, 2)
-    );
-}
+if (!function_exists("calc_image_dissimilarity")) {
+    /**
+     * Calculates the euclidean distance of two RGB values.
+     */
+    function calc_pixel_distance(int $color1, int $color2): float
+    {
+        get_rgb($color1, $red1, $green1, $blue1);
+        get_rgb($color2, $red2, $green2, $blue2);
+        return sqrt(
+            pow($red1 - $red2, 2) + pow($green1 - $green2, 2) + pow($blue1 - $blue2, 2)
+        );
+    }
 
-/**
- * Calculates dissimilarity of two images.
- *
- * @param resource $image1
- * @param resource $image2
- *
- * @return int The dissimilarity. 0 means the images are identical. The higher
- *             the value, the more dissimilar are the images.
- */
-function calc_image_dissimilarity($image1, $image2)
-{
-    // assumes image1 and image2 have same width and height
-    $dissimilarity = 0;
-    for ($i = 0, $n = imagesx($image1); $i < $n; $i++) {
-        for ($j = 0, $m = imagesy($image1); $j < $m; $j++) {
-            $color1 = imagecolorat($image1, $i, $j);
-            $color2 = imagecolorat($image2, $i, $j);
-            $dissimilarity += calc_pixel_distance($color1, $color2);
+    /**
+     * Calculates dissimilarity of two images.
+     *
+     * 0 means the images are identical. The higher
+     * the value, the more dissimilar are the images.
+     */
+    function calc_image_dissimilarity(GdImage $im1, GdImage $im2): float
+    {
+        // assumes image1 and image2 have same width and height
+        $dissimilarity = 0;
+        for ($j = imagesy($im1) - 1; $j >= 0; --$j) {
+            for ($i = imagesx($im1) - 1; $i >= 0; --$i) {
+                $color1 = imagecolorat($im1, $i, $j);
+                $color2 = imagecolorat($im2, $i, $j);
+                $dissimilarity += calc_pixel_distance($color1, $color2);
+            }
         }
+        return $dissimilarity;
     }
-    return $dissimilarity;
 }