GH-16067: prevent invalid abstract during compilation of methods

For classes that are not declared `abstract`, produce a compiler error for any
`abstract` methods. For anonymous classes, since they cannot be made abstract,
the error message is slightly different.

Author: DanielEScherzer

Zend/tests/abstract_implicit.phpt

@@ -16,4 +16,4 @@ class NotAbstract {
 }
 ?>
 --EXPECTF--
-Fatal error: Class NotAbstract contains 1 abstract method and must therefore be declared abstract or implement the remaining method (NotAbstract::bar) in %s on line %d
+Fatal error: Class NotAbstract contains abstract method NotAbstract::bar and must therefore be declared abstract in %s on line %d

Zend/tests/anon/gh16067.phpt

@@ -8,4 +8,4 @@ $c = new class {
 }
 ?>
 --EXPECTF--
-Fatal error: Class class@anonymous must implement 1 abstract method (class@anonymous::f) in %s on line 3
+Fatal error: Anonymous class class@anonymous cannot contain abstract method class@anonymous::f in %s on line 4

Zend/zend_compile.c

@@ -8067,6 +8067,27 @@ static zend_string *zend_begin_method_decl(zend_op_array *op_array, zend_string
 		zend_error(E_COMPILE_WARNING, "Private methods cannot be final as they are never overridden by other classes");
 	}
 
+	if ((fn_flags & ZEND_ACC_ABSTRACT) &&
+		!(ce->ce_flags & (ZEND_ACC_EXPLICIT_ABSTRACT_CLASS|ZEND_ACC_TRAIT)) &&
+		!in_interface
+	) {
+		// Don't say that the class should be declared abstract if it is
+		// anonymous and can't be abstract
+		const char *msg;
+		if (ce->ce_flags & ZEND_ACC_ANON_CLASS) {
+			msg = "Anonymous class %s cannot contain abstract method %s::%s";
+		} else {
+			msg = "Class %s contains abstract method %s::%s and must therefore be declared abstract";
+		}
+		zend_error_noreturn(
+			E_COMPILE_ERROR,
+			msg,
+			ZSTR_VAL(ce->name),
+			ZSTR_VAL(ce->name),
+			ZSTR_VAL(name)
+		);
+	}
+
 	if (in_interface) {
 		if (!(fn_flags & ZEND_ACC_PUBLIC)) {
 			zend_error_noreturn(E_COMPILE_ERROR, "Access type for interface method "