IMAP: Disallow direct new IMAPConnection() construct

Disallows constructing an `IMAPConnection` class directly with `new IMAPConnection` construct, by throwing an `Error` exception if attempted.
`imap_open` is still the only way to create `IMAPConnection` objects.

Author: Ayesh

ext/imap/php_imap.c

@@ -170,6 +170,11 @@ static zend_object* imap_object_create(zend_class_entry* ce) {
 	return zobj;
 }
 
+static zend_function *imap_object_get_constructor(zend_object *zobj) {
+	zend_throw_error(NULL, "Cannot directly construct IMAPConnection, use imap_open() instead");
+	return NULL;
+}
+
 static void imap_object_destroy(zend_object *zobj) {
 	php_imap_object *obj = imap_object_from_zend_object(zobj);
 
@@ -483,6 +488,7 @@ PHP_MINIT_FUNCTION(imap)
 
 	memcpy(&imap_object_handlers, &std_object_handlers, sizeof(zend_object_handlers));
 	imap_object_handlers.offset = XtOffsetOf(php_imap_object, std);
+	imap_object_handlers.get_constructor = imap_object_get_constructor;
 	imap_object_handlers.dtor_obj = imap_object_destroy;
 	imap_object_handlers.clone_obj = NULL;
 

ext/imap/tests/imap_constructor.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Attempt to instantiate an IMAPConnection directly
+--SKIPIF--
+<?php
+extension_loaded('imap') or die('skip imap extension not available in this build');
+--FILE--
+<?php
+
+try {
+    new IMAPConnection();
+} catch (Error $ex) {
+    echo "Exception: ", $ex->getMessage(), "\n";
+}
+--EXPECT--
+Exception: Cannot directly construct IMAPConnection, use imap_open() instead