$d = array(
  0 => array(
    'a' => string 'Days_3' (length=6)
    'b' => string '3 Days' (length=6)
  )
  1 => array(
    'a' => string 'Days_7' (length=6)
    'b' => string '7 Days' (length=6)
  )
  2 => array(
    'a' => string 'Days_14' (length=7)
    'b' => string '14 Days' (length=7)
  )
  3 => array(
    'a' => string 'Days_30' (length=7)
    'b' => string '30 Days' (length=7)
  )
  4 => array(
    'a' => string 'Days_60' (length=7)
    'b' => string '60 Days' (length=7)
  ));
foreach($d as $v)
{
    echo ltrim($v['a'],"Days_").",";
}

$ch = '';
foreach($d as $v)
{
    echo $ch . ltrim($v['a'],"Days_");
    $ch = ',';
}

------解决方案--------------------
substr($str,0,-1)或者放入数组中。再用implode连接