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Try using the built in strtotime function in PHP and using 'F' for full textual output:
echo date('y:F:d'); // first month
echo date('y:F:d', strtotime('-1 month')); // previous month
echo date('y:F:d', strtotime('-2 month')); // second previous month
echo date('y:F:d', strtotime('-3 month')); // third previous month