The behaviour of these functions is affected by settings in php.ini.
Here's a short explanation of the configuration directives.
Sets limit for disposed contexts before forcing V8 to do garbage collection.
Sets V8 command line flags. The list of available flags can be obtained in CLI mode by setting this parameter to --help. Example:
$ php -r 'ini_set("v8js.flags", "--help"); new V8Js;' | less
For these flags to be effective in runtime the ini_set() call has to be done before any V8Js objects are instantiated!