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<?php $input = new \DateTime('2017-07-14'); $firstDayOfMonth = new \DateTime($input->format('Y-m-01')); var_dump($input->format('l')); var_dump($input->format('n') % 7);
Output for 5.6.38, 7.0.0 - 7.0.31, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
string(6) "Friday" int(0)

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