Version difference for strtotime('first day of last month')?

You should use the mktime() function:

<?php 
echo date('Y-m-d', mktime(0,0,0,date('n')-1,1,date('Y'))); //2012-03-01
?>

See In Action

date('Y-m-d', strtotime('first day of -1 month'))

Works fine on PHP 7.0