diff --git a/TSRM/tsrm_win32.c b/TSRM/tsrm_win32.c
index 4c8fc9d19aa9a..ec385596814ed 100644
--- a/TSRM/tsrm_win32.c
+++ b/TSRM/tsrm_win32.c
@@ -86,6 +86,37 @@ static void tsrm_win32_dtor(tsrm_win32_globals *globals)
 	}
 }/*}}}*/
 
+/**
+ * Converts Windows GetLastError() codes to POSIX errno values
+ * @param win32_error
+ */
+static void tsrm_set_errno_from_win32_error(const DWORD win32_error)
+{/*{{{*/
+	switch (win32_error) {
+		case ERROR_ACCESS_DENIED:
+			errno = EACCES;
+			break;
+		case ERROR_NOT_ENOUGH_MEMORY:
+		case ERROR_OUTOFMEMORY:
+			errno = ENOMEM;
+			break;
+		case ERROR_INVALID_PARAMETER:
+		case ERROR_INVALID_HANDLE:
+			errno = EINVAL;
+			break;
+		case ERROR_ALREADY_EXISTS:
+			errno = EEXIST;
+			break;
+		case ERROR_FILE_NOT_FOUND:
+		case ERROR_PATH_NOT_FOUND:
+			errno = ENOENT;
+			break;
+		default:
+			errno = EINVAL;
+			break;
+	}
+}/*}}}*/
+
 TSRM_API void tsrm_win32_startup(void)
 {/*{{{*/
 #ifdef ZTS
@@ -651,6 +682,7 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 	if (!shm_handle) {
 		if (flags & IPC_CREAT) {
 			if (size == 0 || size > SIZE_MAX - sizeof(shm->descriptor)) {
+				errno = EINVAL;
 				return -1;
 			}
 			size += sizeof(shm->descriptor);
@@ -665,11 +697,13 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 			created		= TRUE;
 		}
 		if (!shm_handle) {
+			tsrm_set_errno_from_win32_error(GetLastError());
 			return -1;
 		}
 	} else {
 		if (flags & IPC_EXCL) {
 			CloseHandle(shm_handle);
+			errno = EEXIST;
 			return -1;
 		}
 	}
@@ -690,6 +724,7 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 	shm = shm_get(key, NULL);
 	if (!shm) {
 		CloseHandle(shm_handle);
+		errno = ENOMEM;
 		return -1;
 	}
 	shm->segment = shm_handle;
@@ -716,6 +751,7 @@ TSRM_API int shmget(key_t key, size_t size, int flags)
 		}
 		UnmapViewOfFile(shm->descriptor);
 		shm->descriptor = NULL;
+		errno = EINVAL;
 		return -1;
 	}
 
@@ -744,6 +780,7 @@ TSRM_API int shmdt(const void *shmaddr)
 	int ret;
 
 	if (!shm || !shm->segment) {
+		errno = EINVAL;
 		return -1;
 	}
 
@@ -753,7 +790,10 @@ TSRM_API int shmdt(const void *shmaddr)
 
 	ret = 0;
 	if (shm->descriptor->shm_nattch <= 0) {
-		ret = UnmapViewOfFile(shm->descriptor) ? 0 : -1;
+		if (!UnmapViewOfFile(shm->descriptor)) {
+			tsrm_set_errno_from_win32_error(GetLastError());
+			ret = -1;
+		}
 		shm->descriptor = NULL;
 	}
 	return ret;
@@ -764,6 +804,7 @@ TSRM_API int shmctl(int key, int cmd, struct shmid_ds *buf)
 	shm_pair *shm = shm_get(key, NULL);
 
 	if (!shm || !shm->segment) {
+		errno = EINVAL;
 		return -1;
 	}
 
@@ -782,10 +823,16 @@ TSRM_API int shmctl(int key, int cmd, struct shmid_ds *buf)
 		case IPC_RMID:
 			if (shm->descriptor->shm_nattch < 1) {
 				shm->descriptor->shm_perm.key = -1;
+				/* Close handle to allow Windows to destroy the named mapping object */
+				if (shm->segment && shm->segment != INVALID_HANDLE_VALUE) {
+					CloseHandle(shm->segment);
+					shm->segment = INVALID_HANDLE_VALUE;
+				}
 			}
 			return 0;
 
 		default:
+			errno = EINVAL;
 			return -1;
 	}
 }/*}}}*/
diff --git a/Zend/zend.h b/Zend/zend.h
index e2e500022a85e..08ee5f89d4619 100644
--- a/Zend/zend.h
+++ b/Zend/zend.h
@@ -20,7 +20,7 @@
 #ifndef ZEND_H
 #define ZEND_H
 
-#define ZEND_VERSION "4.5.1-dev"
+#define ZEND_VERSION "4.5.1"
 
 #define ZEND_ENGINE_3
 
diff --git a/configure.ac b/configure.ac
index f81d8cf609054..49f0fd191a205 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,7 +17,7 @@ dnl Basic autoconf initialization, generation of config.nice.
 dnl ----------------------------------------------------------------------------
 
 AC_PREREQ([2.68])
-AC_INIT([PHP],[8.5.1-dev],[https://github.com/php/php-src/issues],[php],[https://www.php.net])
+AC_INIT([PHP],[8.5.1],[https://github.com/php/php-src/issues],[php],[https://www.php.net])
 AC_CONFIG_SRCDIR([main/php_version.h])
 AC_CONFIG_AUX_DIR([build])
 AC_PRESERVE_HELP_ORDER
diff --git a/ext/shmop/tests/shmop_errno_codes_win32.phpt b/ext/shmop/tests/shmop_errno_codes_win32.phpt
new file mode 100644
index 0000000000000..410571563f26e
--- /dev/null
+++ b/ext/shmop/tests/shmop_errno_codes_win32.phpt
@@ -0,0 +1,21 @@
+--TEST--
+shmop errno codes on Windows
+--EXTENSIONS--
+shmop
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== "Windows") die("skip for Windows only");
+?>
+--FILE--
+<?php
+$key = 0xABCDEF01;
+$shm1 = shmop_open($key, 'n', 0644, 1024);
+if ($shm1 !== false) {
+    $shm2 = shmop_open($key, 'n', 0644, 1024);
+    shmop_delete($shm1);
+}
+echo "done\n";
+?>
+--EXPECTF--
+Warning: shmop_open(): Unable to attach or create shared memory segment "File exists" in %s on line %d
+done
diff --git a/ext/shmop/tests/shmop_errno_mapping_win32.phpt b/ext/shmop/tests/shmop_errno_mapping_win32.phpt
new file mode 100644
index 0000000000000..7c8f57ea1b7c5
--- /dev/null
+++ b/ext/shmop/tests/shmop_errno_mapping_win32.phpt
@@ -0,0 +1,40 @@
+--TEST--
+shmop Windows errno mapping
+--EXTENSIONS--
+shmop
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== "Windows") die("skip for Windows only");
+?>
+--FILE--
+<?php
+echo "EEXIST test:\n";
+$key = 0xEEEEEEEE;
+$shm1 = shmop_open($key, 'n', 0644, 512);
+if ($shm1 !== false) {
+    $shm2 = shmop_open($key, 'n', 0644, 512);
+    shmop_delete($shm1);
+}
+
+echo "\nENOENT attach test:\n";
+$shm = shmop_open(0x99999999, 'a', 0644, 0);
+
+echo "\nENOENT write test:\n";
+$shm = shmop_open(0x77777777, 'w', 0644, 0);
+
+echo "\ndone\n";
+?>
+--EXPECTF--
+EEXIST test:
+
+Warning: shmop_open(): Unable to attach or create shared memory segment "File exists" in %s on line %d
+
+ENOENT attach test:
+
+Warning: shmop_open(): Unable to attach or create shared memory segment "No such file or directory" in %s on line %d
+
+ENOENT write test:
+
+Warning: shmop_open(): Unable to attach or create shared memory segment "No such file or directory" in %s on line %d
+
+done
diff --git a/ext/shmop/tests/shmop_errno_tests.phpt b/ext/shmop/tests/shmop_errno_tests.phpt
new file mode 100644
index 0000000000000..da80c26b8476d
--- /dev/null
+++ b/ext/shmop/tests/shmop_errno_tests.phpt
@@ -0,0 +1,65 @@
+--TEST--
+shmop errno handling tests
+--EXTENSIONS--
+shmop
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== "Windows") die("skip for Windows only");
+?>
+--FILE--
+<?php
+$key = 0x12345678;
+$size = 1024;
+
+echo "create segment: ";
+$shm1 = shmop_open($key, 'n', 0644, $size);
+if ($shm1 === false) {
+    die("failed\n");
+}
+echo "ok\n";
+
+echo "duplicate with IPC_EXCL: ";
+$shm2 = shmop_open($key, 'n', 0644, $size);
+if ($shm2 === false) {
+    echo "ok\n";
+} else {
+    echo "failed\n";
+    shmop_delete($shm2);
+}
+shmop_delete($shm1);
+
+echo "attach non-existent: ";
+$shm = shmop_open(0x99999999, 'a', 0644, 0);
+echo ($shm === false) ? "ok\n" : "failed\n";
+
+echo "write mode non-existent: ";
+$shm = shmop_open(0x88888888, 'w', 0644, 0);
+echo ($shm === false) ? "ok\n" : "failed\n";
+
+echo "create/attach/write sequence: ";
+$key = 0x45678901;
+$shm_c = shmop_open($key, 'c', 0644, 512);
+$shm_a = shmop_open($key, 'a', 0644, 0);
+$shm_w = shmop_open($key, 'w', 0644, 0);
+if ($shm_c && $shm_a && $shm_w) {
+    $data = "test";
+    $written = shmop_write($shm_w, $data, 0);
+    $read = shmop_read($shm_a, 0, strlen($data));
+    echo ($read === $data) ? "ok\n" : "failed\n";
+    shmop_delete($shm_c);
+} else {
+    echo "failed\n";
+}
+?>
+--EXPECTF--
+create segment: ok
+duplicate with IPC_EXCL: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+attach non-existent: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+write mode non-existent: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+create/attach/write sequence: ok
diff --git a/ext/shmop/tests/shmop_error_conditions_win32.phpt b/ext/shmop/tests/shmop_error_conditions_win32.phpt
new file mode 100644
index 0000000000000..d60ad74dc5351
--- /dev/null
+++ b/ext/shmop/tests/shmop_error_conditions_win32.phpt
@@ -0,0 +1,82 @@
+--TEST--
+shmop error conditions on Windows
+--EXTENSIONS--
+shmop
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== "Windows") die("skip for Windows only");
+?>
+--FILE--
+<?php
+echo "duplicate segment: ";
+$key = 0xFEEDBEEF;
+$shm1 = shmop_open($key, 'n', 0644, 1024);
+$shm2 = shmop_open($key, 'n', 0644, 1024);
+echo ($shm1 && !$shm2) ? "ok\n" : "failed\n";
+shmop_delete($shm1);
+
+echo "non-existent attach: ";
+$shm = shmop_open(0xDEADC0DE, 'a', 0644, 0);
+echo ($shm === false) ? "ok\n" : "failed\n";
+
+echo "non-existent write: ";
+$shm = shmop_open(0xBADF00D, 'w', 0644, 0);
+echo ($shm === false) ? "ok\n" : "failed\n";
+
+echo "zero size: ";
+try {
+    shmop_open(0x12340000, 'n', 0644, 0);
+    echo "failed\n";
+} catch (ValueError $e) {
+    echo "ok\n";
+}
+
+echo "write/read test: ";
+$key = 0xC0FFEE00;
+$shm_c = shmop_open($key, 'c', 0644, 1024);
+$shm_w = shmop_open($key, 'w', 0644, 0);
+$shm_a = shmop_open($key, 'a', 0644, 0);
+if ($shm_c && $shm_w && $shm_a) {
+    $data = "test data";
+    $written = shmop_write($shm_w, $data, 0);
+    $read = shmop_read($shm_a, 0, strlen($data));
+    echo ($read === $data) ? "ok\n" : "failed\n";
+    shmop_delete($shm_c);
+} else {
+    echo "failed\n";
+}
+
+echo "multiple segments: ";
+$segments = [];
+for ($i = 0; $i < 3; $i++) {
+    $shm = shmop_open(0xA0000000 + $i, 'c', 0644, 256);
+    if ($shm) $segments[] = $shm;
+}
+echo (count($segments) === 3) ? "ok\n" : "failed\n";
+foreach ($segments as $shm) {
+    shmop_delete($shm);
+}
+
+echo "large segment: ";
+$shm = shmop_open(0xBEEF0000, 'n', 0644, 1024 * 1024);
+if ($shm) {
+    echo (shmop_size($shm) >= 1024 * 1024) ? "ok\n" : "failed\n";
+    shmop_delete($shm);
+} else {
+    echo "failed\n";
+}
+?>
+--EXPECTF--
+duplicate segment: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+non-existent attach: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+non-existent write: 
+Warning: shmop_open(): Unable to attach or create shared memory segment%s in %s on line %d
+ok
+zero size: ok
+write/read test: ok
+multiple segments: ok
+large segment: ok
diff --git a/main/php_version.h b/main/php_version.h
index 87cf375f2e0f9..774a31a7d8f53 100644
--- a/main/php_version.h
+++ b/main/php_version.h
@@ -3,6 +3,6 @@
 #define PHP_MAJOR_VERSION 8
 #define PHP_MINOR_VERSION 5
 #define PHP_RELEASE_VERSION 1
-#define PHP_EXTRA_VERSION "-dev"
-#define PHP_VERSION "8.5.1-dev"
+#define PHP_EXTRA_VERSION ""
+#define PHP_VERSION "8.5.1"
 #define PHP_VERSION_ID 80501