Be aware of this: 1 month before the 31st day, it will return the same month:
<?php
echo date('m', strtotime('2023-05-30 -1 month')) ; //returns 04
echo date('m', strtotime('2023-05-31 -1 month')) ; //returns 05, not 04
?>
So, don't use this to operate on the month of the result.
A better way to know what month was the previous month would be:
<?php
//considering today is 2023-05-31...
$firstOfThisMonth = date('Y-m') . '-01'; //returns 2023-05-01
echo date('m', strtotime($firstOfThisMonth . ' -1 month')) ; //returns 04
?>