GH-18463: skip recursion protection for internal constants

Internal constants are persisted; if a user error handler triggers bailout
before the recursion flag is removed then a subsequent request (e.g. with
`--repeat 2`) will start with that flag already applied.

Internal constants can presumably be trusted not to use deprecation messages
that come from recursive attributes.

Author: DanielEScherzer

Zend/tests/constants/gh18463-class-constant.phpt

@@ -0,0 +1,20 @@
+--TEST--
+GH-18463: Recursion protection should not be applied to internal class constants
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+
+function handler($errno, $errstr, $errfile, $errline) {
+	echo "$errstr in $errfile on line $errline\n";
+	eval('class string {}');
+}
+
+set_error_handler('handler');
+
+var_dump(_ZendTestClass::ZEND_TEST_DEPRECATED);
+?>
+--EXPECTF--
+Constant _ZendTestClass::ZEND_TEST_DEPRECATED is deprecated in %s on line %d
+
+Fatal error: Cannot use "string" as a class name as it is reserved in %s(%d) : eval()'d code on line %d

Zend/zend_constants.h

@@ -32,11 +32,17 @@
 #define CONST_IS_RECURSIVE(c) (Z_CONSTANT_FLAGS((c)->value) & CONST_RECURSIVE)
 #define CONST_PROTECT_RECURSION(c) \
 	do { \
-		Z_CONSTANT_FLAGS((c)->value) |= CONST_RECURSIVE; \
+		/* Ignored for internal constants, GH-18463 */ \
+		if (ZEND_CONSTANT_MODULE_NUMBER(c) == PHP_USER_CONSTANT) { \
+			Z_CONSTANT_FLAGS((c)->value) |= CONST_RECURSIVE; \
+		} \
 	} while (0)
 #define CONST_UNPROTECT_RECURSION(c) \
 	do { \
-		Z_CONSTANT_FLAGS((c)->value) &= ~CONST_RECURSIVE; \
+		/* Ignored for internal constants, GH-18463 */ \
+		if (ZEND_CONSTANT_MODULE_NUMBER(c) == PHP_USER_CONSTANT) { \
+			Z_CONSTANT_FLAGS((c)->value) &= ~CONST_RECURSIVE; \
+		} \
 	} while (0)
 
 #define	PHP_USER_CONSTANT   0x7fffff /* a constant defined in user space */