<?php
$m = (integer) date('n');

$start = date('Y-m-d',mktime(1,1,1,$m,1,date('Y')));
$end = date('Y-m-d',mktime(1,1,1,++$m,0,date('Y'))); 
echo $start;
echo "\n";
echo $end;

2014-02-01
2014-02-28