Fix return type of mysqli::init()

Author: kocsismate

ext/mysqli/mysqli.stub.php

@@ -113,7 +113,7 @@ public function get_server_info() {}
     public function get_warnings() {}
 
     /**
-     * @return mysqli|false
+     * @return null|false
      */
     public function init() {}
 

ext/mysqli/mysqli_arginfo.h

@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 36c7ee22aa4c5bb5e0174031a3defcaaadde0dbd */
+ * Stub hash: c2ab263780902447855ecb0d8a9e3432577a32d1 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_MASK_EX(arginfo_mysqli_affected_rows, 0, 1, MAY_BE_LONG|MAY_BE_STRING)
 	ZEND_ARG_OBJ_INFO(0, mysql, mysqli, 0)