ext/standard: Deprecate calling class_exists with Enums

Calling `class_exists()` with an Enum name emits a deprecation
notice:

```
Deprecated: using class_exists() for enums is deprecated,
triggerred for calling class_exists() for enum "Foo". Use
enum_exists() instead %s on line %d
```

Author: Ayesh

UPGRADING

@@ -548,6 +548,8 @@ PHP 8.4 UPGRADE NOTES
     - str_getcsv()
     is now deprecated.
     RFC: https://wiki.php.net/rfc/deprecations_php_8_4#deprecate_proprietary_csv_escaping_mechanism
+  . Calling class_exists() with an enum name is deprecated in favor of the
+    enum_exists() function.
 
 - XML:
   . The xml_set_object() function has been deprecated.

Zend/tests/enum/class_exists-enums-deprecated.phpt

@@ -0,0 +1,75 @@
+--TEST--
+Calling class_exists() on Enums should trigger a deprecation
+--DESCRIPTION--
+Calling class_exists() on Enums is deprecated, but it should continue to return
+the correct result. The underlying functionality for (class|trait|enum|interface)_exists
+is shared, so this deprecation should only affect class_exists() function and no
+other functions.
+--FILE--
+<?php
+
+enum Foo {
+    case Bar;
+}
+
+class Bar {
+
+}
+
+spl_autoload_register(function ($className) {
+    echo "Triggered autoloader with Enum $className\n";
+
+    if ($className === 'Quux') {
+        enum Quux {}
+    }
+});
+
+echo "Testing: Foo";
+var_dump(class_exists('Foo'));
+var_dump(enum_exists('Foo'));
+
+echo "Testing: Bar\n";
+var_dump(class_exists('Bar'));
+var_dump(!enum_exists('Bar'));
+var_dump(!enum_exists('Bar', true));
+
+echo "Testing: Quux\n";
+var_dump(!class_exists('Quux', false));
+var_dump(class_exists('Quux', true));
+var_dump(class_exists('Quux', true));
+var_dump(enum_exists('Quux', true));
+
+echo "trait_exists() and interface_exists()\n";
+var_dump(!trait_exists('Foo'));
+var_dump(!trait_exists('Bar'));
+var_dump(!trait_exists('Quux'));
+var_dump(!interface_exists('Foo'));
+var_dump(!interface_exists('Bar'));
+var_dump(!interface_exists('Quux'));
+?>
+--EXPECTF--
+Testing: Foo
+Deprecated: using class_exists() for enums is deprecated, triggerred for calling class_exists() for enum "Foo". Use enum_exists() instead %s on line %d
+bool(true)
+bool(true)
+Testing: Bar
+bool(true)
+bool(true)
+bool(true)
+Testing: Quux
+bool(true)
+Triggered autoloader with Enum Quux
+
+Deprecated: using class_exists() for enums is deprecated, triggerred for calling class_exists() for enum "Quux". Use enum_exists() instead %s on line %d
+bool(true)
+
+Deprecated: using class_exists() for enums is deprecated, triggerred for calling class_exists() for enum "Quux". Use enum_exists() instead %s on line %d
+bool(true)
+bool(true)
+trait_exists() and interface_exists()
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)

Zend/zend_builtin_functions.c

@@ -1061,6 +1061,9 @@ static inline void _class_exists_impl(zval *return_value, zend_string *name, boo
 	if (ZSTR_HAS_CE_CACHE(name)) {
 		ce = ZSTR_GET_CE_CACHE(name);
 		if (ce) {
+			if (flags == ZEND_ACC_LINKED && ce->ce_flags & ZEND_ACC_ENUM) {
+				zend_error(E_DEPRECATED, "using class_exists() for enums is deprecated, triggerred for calling class_exists() for enum \"%s\". Use enum_exists() instead", ZSTR_VAL(ce->name));
+			}
 			RETURN_BOOL(((ce->ce_flags & flags) == flags) && !(ce->ce_flags & skip_flags));
 		}
 	}
@@ -1081,6 +1084,9 @@ static inline void _class_exists_impl(zval *return_value, zend_string *name, boo
 	}
 
 	if (ce) {
+		if (flags == ZEND_ACC_LINKED && ce->ce_flags & ZEND_ACC_ENUM) {
+    		zend_error(E_DEPRECATED, "using class_exists() for enums is deprecated, triggerred for calling class_exists() for enum \"%s\". Use enum_exists() instead", ZSTR_VAL(ce->name));
+    	}
 		RETURN_BOOL(((ce->ce_flags & flags) == flags) && !(ce->ce_flags & skip_flags));
 	} else {
 		RETURN_FALSE;