diff --git a/NEWS b/NEWS
index 2efb946fded..6493d92179d 100644
--- a/NEWS
+++ b/NEWS
@@ -32,6 +32,9 @@ PHP                                                                        NEWS
   . Fixed bug #81208 (Segmentation fault while create newInstance from
     attribute). (Nikita)
 
+- Standard:
+  . Fixed bug #81223 (flock() only locks first byte of file). (cmb)
+
 17 Jun 2021, PHP 8.0.8
 
 - Core:
diff --git a/ext/standard/flock_compat.c b/ext/standard/flock_compat.c
index 7b0ad2df23c..da49888b56c 100644
--- a/ext/standard/flock_compat.c
+++ b/ext/standard/flock_compat.c
@@ -103,7 +103,7 @@ PHPAPI int php_flock(int fd, int operation)
  */
 {
     HANDLE hdl = (HANDLE) _get_osfhandle(fd);
-    DWORD low = 1, high = 0;
+    DWORD low = 0xFFFFFFFF, high = 0xFFFFFFFF;
     OVERLAPPED offset =
     {0, 0, 0, 0, NULL};
 	DWORD err;
diff --git a/ext/standard/tests/file/bug81223.phpt b/ext/standard/tests/file/bug81223.phpt
new file mode 100644
index 00000000000..0331d631bc8
--- /dev/null
+++ b/ext/standard/tests/file/bug81223.phpt
@@ -0,0 +1,24 @@
+--TEST--
+Bug #81223 (flock() only locks first byte of file)
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== "Windows") die("skip for Windows only");
+?>
+--FILE--
+<?php
+$filename = __FILE__;
+$stream1 = fopen($filename, "r");
+var_dump(flock($stream1, LOCK_EX));
+$stream2 = fopen($filename, "r");
+var_dump(fread($stream2, 5));
+fseek($stream2, 1);
+var_dump(fread($stream2, 4));
+?>
+--EXPECTF--
+bool(true)
+
+Notice: fread(): read of %d bytes failed with errno=13 Permission denied in %s on line %d
+bool(false)
+
+Notice: fread(): read of %d bytes failed with errno=13 Permission denied in %s on line %d
+bool(false)