$t1 = strtotime('2013-07-01 00:00:00');
$t2 = strtotime('2013-07-10 23:59:59');
$s  = strtotime('2013-07-05 00:00:00');
$e  = strtotime('2013-07-15 23:59:59');

$xs = date( 'Y-m-d H:i:s', $t1 > $s ? $t1 : $s );
$xe = date( 'Y-m-d H:i:s', $t2 < $e ? $t2 : $e );
if( ($s >= $t1 && $s < = $t2) || ($t1 >= $s && $t1 < = $e) ){
	echo $xs, ' ', $xe, "\r\n";
}

$xs = $t1 > $s ? $t1 : $s ;
$xe = $t2 < $e ? $t2 : $e ;
if($xs <= $xe){
	echo date('Y-m-d H:i:s', $xs), ' ', date('Y-m-d H:i:s', $xe), "\r\n";
}