Fatal error: Call to undefined function imagegrabwindow()

I am tryiong to work with imagegrabscreen() in php(in Windows).Uncomment php_gd2.dll in php.ini and check "Interact with the desktop" in services but it shows an error like
Fatal error: Call to undefined function imagegrabwindow() .
Please help

Thanks

$im = imagegrabscreen();
 imagepng($im,"myscreenshot.png");

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