Description:
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The PHP version is actually 7.0.30 (not in the list)

The test script produces the following result:
2018-07-31 
2018-07-01 
2018-06-01


Test script:
---------------
<?php
$now = new DateTime('2018-07-31');
echo $now->format('Y-m-d') . "\n";
echo $now->sub(new DateInterval('P1M'))->format('Y-m-d') . "\n";
echo $now->sub(new DateInterval('P1M'))->format('Y-m-d') . "\n";