<?php

function getFileName($unixTime) {
    return 'abcd_' . date('Y_m_j', $unixTime) . '.txt';
} 

$files = array();

foreach(range(0, 6) as $dayOffset) {

   $files[] = getFileName(strtotime('-' . $dayOffset . ' day'));
}

var_dump($files);

array(7) {
  [0]=>
  string(19) "abcd_2011_04_21.txt"
  [1]=>
  string(19) "abcd_2011_04_20.txt"
  [2]=>
  string(19) "abcd_2011_04_19.txt"
  [3]=>
  string(19) "abcd_2011_04_18.txt"
  [4]=>
  string(19) "abcd_2011_04_17.txt"
  [5]=>
  string(19) "abcd_2011_04_16.txt"
  [6]=>
  string(19) "abcd_2011_04_15.txt"
}