diff --git a/ext/standard/tests/file/bug81145.phpt b/ext/standard/tests/file/bug81145.phpt
new file mode 100644
index 0000000000000..2f12d9eee81da
--- /dev/null
+++ b/ext/standard/tests/file/bug81145.phpt
@@ -0,0 +1,48 @@
+--TEST--
+Bug #81145 (copy() and stream_copy_to_stream() fail for +4GB files)
+--SKIPIF--
+<?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+if (PHP_INT_SIZE !== 8) die("skip this test is for 64bit platforms only");
+if (disk_free_space(__DIR__) < 0x220000000) die("skip insuffient disk space");
+if (PHP_OS_FAMILY !== "Windows") {
+    exec("fallocate -h", $output, $status);
+    if ($status !== 0) die("skip fallocate(1) not available");
+}
+?>
+--FILE--
+<?php
+$src = __DIR__ . "/bug81145_src.bin";
+$dst = __DIR__ . "/bug81145_dst.bin";
+define('SIZE_4G', 0x100000000);
+
+//Create file and append random content at the 4GB boundary
+if (PHP_OS_FAMILY !== "Windows") {
+    exec("fallocate -l " . (SIZE_4G-0x100) . " " . escapeshellarg($src));
+} else {
+    exec("fsutil file createnew " . escapeshellarg($src) . " " . (SIZE_4G-0x100));
+}
+$fp = fopen($src, "ab");
+fwrite($fp, random_bytes(0x200));
+fclose($fp);
+copy($src, $dst);
+if (filesize($src) !== filesize($dst)) {
+    die("Files have different sizes!");
+}
+$f1 = fopen($src,'rb') or die("src open failed");
+$f2 = fopen($dst,'rb') or die("dst open failed");
+
+//Seek to 4 GB boundary, as this is the location where the problem occurs
+fseek($f1, SIZE_4G - 0x100, SEEK_SET);
+fseek($f2, SIZE_4G - 0x100, SEEK_SET);
+echo (fread($f1,0x200) === fread($f2,0x200) ? "Identical" : "Copy failed");
+fclose($f1);
+fclose($f2);
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . "/bug81145_src.bin");
+@unlink(__DIR__ . "/bug81145_dst.bin");
+?>
+--EXPECT--
+Identical
diff --git a/main/streams/plain_wrapper.c b/main/streams/plain_wrapper.c
index d187c23a59316..4d10e688b5f76 100644
--- a/main/streams/plain_wrapper.c
+++ b/main/streams/plain_wrapper.c
@@ -770,7 +770,7 @@ static int php_stdiop_set_option(php_stream *stream, int option, int value, void
 			{
 				php_stream_mmap_range *range = (php_stream_mmap_range*)ptrparam;
 				HANDLE hfile = (HANDLE)_get_osfhandle(fd);
-				DWORD prot, acc, loffs = 0, delta = 0;
+				DWORD prot, acc, loffs = 0, hoffs = 0, delta = 0;
 				LARGE_INTEGER file_size;
 
 				switch (value) {
@@ -838,8 +838,11 @@ static int php_stdiop_set_option(php_stream *stream, int option, int value, void
 
 							GetSystemInfo(&info);
 							gran = info.dwAllocationGranularity;
-							loffs = ((DWORD)range->offset / gran) * gran;
-							delta = (DWORD)range->offset - loffs;
+							ZEND_ASSERT(gran != 0 && (gran & (gran - 1)) == 0);
+							size_t rounded_offset = (range->offset / gran) * gran;
+							delta = range->offset - rounded_offset;
+							loffs = (DWORD)rounded_offset;
+							hoffs = (DWORD)(rounded_offset >> 32);
 						}
 
 						/* MapViewOfFile()ing zero bytes would map to the end of the file; match *nix behavior instead */
@@ -847,7 +850,7 @@ static int php_stdiop_set_option(php_stream *stream, int option, int value, void
 							return PHP_STREAM_OPTION_RETURN_ERR;
 						}
 
-						data->last_mapped_addr = MapViewOfFile(data->file_mapping, acc, 0, loffs, range->length + delta);
+						data->last_mapped_addr = MapViewOfFile(data->file_mapping, acc, hoffs, loffs, range->length + delta);
 
 						if (data->last_mapped_addr) {
 							/* give them back the address of the start offset they requested */