diff --git a/ext/session/session.c b/ext/session/session.c
index fe39381950b44..3c875296c3a17 100644
--- a/ext/session/session.c
+++ b/ext/session/session.c
@@ -1083,9 +1083,10 @@ PHPAPI int php_session_register_module(const ps_module *ptr) /* {{{ */
 /* }}} */
 
 /* Dummy PS module function */
-/* We consider any ID valid, so we return FAILURE to indicate that a session doesn't exist */
+/* We consider any ID valid (thus also implying that a session with such an ID exists),
+	thus we always return SUCCESS */
 PHPAPI int php_session_validate_sid(PS_VALIDATE_SID_ARGS) {
-	return FAILURE;
+	return SUCCESS;
 }
 
 /* Dummy PS module function */
@@ -2255,18 +2256,24 @@ PHP_FUNCTION(session_regenerate_id)
 		}
 		RETURN_THROWS();
 	}
-	if (PS(use_strict_mode) && PS(mod)->s_validate_sid &&
-		PS(mod)->s_validate_sid(&PS(mod_data), PS(id)) == SUCCESS) {
-		zend_string_release_ex(PS(id), 0);
-		PS(id) = PS(mod)->s_create_sid(&PS(mod_data));
-		if (!PS(id)) {
-			PS(mod)->s_close(&PS(mod_data));
-			PS(session_status) = php_session_none;
-			if (!EG(exception)) {
-				zend_throw_error(NULL, "Failed to create session ID by collision: %s (path: %s)", PS(mod)->s_name, PS(save_path));
+	if (PS(use_strict_mode)) {
+		if ((!PS(mod_user_implemented) && PS(mod)->s_validate_sid) || !Z_ISUNDEF(PS(mod_user_names).name.ps_validate_sid)) {
+			int limit = 3;
+			/* Try to generate non-existing ID */
+			while (limit-- && PS(mod)->s_validate_sid(&PS(mod_data), PS(id)) == SUCCESS) {
+				zend_string_release_ex(PS(id), 0);
+				PS(id) = PS(mod)->s_create_sid(&PS(mod_data));
+				if (!PS(id)) {
+					PS(mod)->s_close(&PS(mod_data));
+					PS(session_status) = php_session_none;
+					if (!EG(exception)) {
+						zend_throw_error(NULL, "Failed to create session ID by collision: %s (path: %s)", PS(mod)->s_name, PS(save_path));
+					}
+					RETURN_THROWS();
+				}
 			}
-			RETURN_THROWS();
 		}
+		// TODO warn that ID cannot be verified? else { }
 	}
 	/* Read is required to make new session data at this point. */
 	if (PS(mod)->s_read(&PS(mod_data), PS(id), &data, PS(gc_maxlifetime)) == FAILURE) {
@@ -2293,7 +2300,6 @@ PHP_FUNCTION(session_regenerate_id)
 /* }}} */
 
 /* {{{ Generate new session ID. Intended for user save handlers. */
-/* This is not used yet */
 PHP_FUNCTION(session_create_id)
 {
 	zend_string *prefix = NULL, *new_id;
@@ -2317,7 +2323,7 @@ PHP_FUNCTION(session_create_id)
 		int limit = 3;
 		while (limit--) {
 			new_id = PS(mod)->s_create_sid(&PS(mod_data));
-			if (!PS(mod)->s_validate_sid) {
+			if (!PS(mod)->s_validate_sid || (PS(mod_user_implemented) && Z_ISUNDEF(PS(mod_user_names).name.ps_validate_sid))) {
 				break;
 			} else {
 				/* Detect collision and retry */
diff --git a/ext/session/tests/gh9583-extra.phpt b/ext/session/tests/gh9583-extra.phpt
new file mode 100644
index 0000000000000..4332dd9b3eb12
--- /dev/null
+++ b/ext/session/tests/gh9583-extra.phpt
@@ -0,0 +1,49 @@
+--TEST--
+GH-9583: session_create_id() fails with user defined save handler that doesn't have a validateId() method
+--EXTENSIONS--
+session
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--FILE--
+<?php
+
+class SessionHandlerTester implements \SessionHandlerInterface
+{
+
+    public function close(): bool { return true; }
+
+    public function destroy($id): bool { return true; }
+
+    public function gc($max_lifetime): int|false { return 1; }
+
+    public function open($path, $name): bool { return true; }
+
+    public function read($id): string { return ''; }
+
+    public function write($id, $data): bool { return true; }
+
+    //public function create_sid() { return uniqid(); }
+
+    //public function validateId($key) { return true; }
+}
+
+$obj = new SessionHandlerTester();
+ini_set('session.use_strict_mode','1');
+session_set_save_handler($obj);
+session_start();
+
+$originalSessionId = session_id();
+
+session_write_close();
+
+session_start();
+
+$newSessionId = session_id();
+
+echo 'validateId() ', (method_exists($obj, 'validateId') ? ('returns ' . ($obj->validateId(1) ? 'true' : 'false')) : 'is commented out'), "\n";
+var_dump($originalSessionId == $newSessionId);
+
+?>
+--EXPECT--
+validateId() is commented out
+bool(true)