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<?php $a = strtotime(date('c')); $b = strtotime(date('c', time() + 60*15)); var_dump($b-$a);
Output for 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.25, 7.2.0 - 7.2.13, 7.3.0 - 7.3.1
int(900)
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9
int(0)

preferences:
139.62 ms | 402 KiB | 232 Q