[RFC] Deprecate is_a() and is_subclass_of() with string when not allowed (#23236)

For `is_a()` and `is_subclass_of()`, emit deprecation warnings when calling
with a string as the first argument but with `$allow_string` being false. The
two functions share implementation code and so are updated together.

https://wiki.php.net/rfc/deprecations_php_8_6

Author: DanielEScherzer

Zend/tests/is_a_string.phpt

@@ -0,0 +1,16 @@
+--TEST--
+is_a() with $allow_string false and a string
+--FILE--
+<?php
+
+class Demo {}
+
+var_dump(is_a(Demo::class, Demo::class, true));
+var_dump(is_a(Demo::class, Demo::class, false));
+
+?>
+--EXPECTF--
+bool(true)
+
+Deprecated: Calling is_a() with a string when $allow_string is false in %s on line %d
+bool(false)

Zend/tests/is_subclass_of_string.phpt

@@ -0,0 +1,18 @@
+--TEST--
+is_subclass_of() with $allow_string false and a string
+--FILE--
+<?php
+
+class Demo {}
+
+class Child extends Demo {}
+
+var_dump(is_subclass_of(Child::class, Demo::class, true));
+var_dump(is_subclass_of(Child::class, Demo::class, false));
+
+?>
+--EXPECTF--
+bool(true)
+
+Deprecated: Calling is_subclass_of() with a string when $allow_string is false in %s on line %d
+bool(false)

Zend/zend_builtin_functions.c

@@ -714,6 +714,18 @@ static void is_a_impl(INTERNAL_FUNCTION_PARAMETERS, bool only_subclass) /* {{{ *
 		if (!instance_ce) {
 			RETURN_FALSE;
 		}
+	} else if (Z_TYPE_P(obj) == IS_STRING) {
+		// is_a() uses only_subclass as false, is_subclass_of() uses it as true
+		zend_error(
+			E_DEPRECATED,
+			only_subclass
+				? "Calling is_subclass_of() with a string when $allow_string is false"
+				: "Calling is_a() with a string when $allow_string is false"
+		);
+		if (UNEXPECTED(EG(exception))) {
+			RETURN_THROWS();
+		}
+		RETURN_FALSE;
 	} else if (Z_TYPE_P(obj) == IS_OBJECT) {
 		instance_ce = Z_OBJCE_P(obj);
 	} else {