Listing chooser

chooser.php

<?php
/*
This script takes URL parameters language_path, category, year and month and
assigns to variables $p, $c, $y and $m. It is assumed language_path and category
will consist of alphanumeric or underscore/hyphen characters, and the year and
month will be integers.
The user is redirected to the appropriate predictable archive listing URL if it
exists, otherwise user is redirected to a "no results" page.
TO DO: If user specifies a month but no year, currently no "invalid" message is
displayed, but rather we just redirect to the "no results" page.
*/

$p = isset($_GET['language_path']) ? preg_replace('/[^a-z0-1-_]/i','',$_GET['language_path'])  : ''    ;
$c = isset($_GET['category'])      ? preg_replace('/[^a-z0-1-_]/i','',$_GET['category'])       : false ;
$y = isset($_GET['year'])          ? strval(intval($_GET['year']))                             : false ;
$m = isset($_GET['month'])         ? intval($_GET['month'])                                    : false ;

$blog_url  = str_replace('/main/','/main/'.$p,'/<$mt:BlogURL$>');
$blog_path = str_replace('/main/','/main/'.$p,'/<$mt:BlogSitePath$>');
$params    = '?category='.$c.'&year='.str_pad($y,4,'0',STR_PAD_LEFT).'&month='.str_pad($m,2,'0',STR_PAD_LEFT);
$options   = '';

       if( $c && $y && $m ){
  $options = $c.'/'.str_pad($y,4,'0',STR_PAD_LEFT).'/'.str_pad($m,2,'0',STR_PAD_LEFT);
} else if( $c && $y       ){
  $options = $c.'/'.str_pad($y,4,'0',STR_PAD_LEFT);
} else if( $c       && $m ){
  $params .= '&nonefound';
} else if( $c             ){
  $options = $c;
} else if(       $y && $m ){
  $options = str_pad($y,4,'0',STR_PAD_LEFT).'/'.str_pad($m,2,'0',STR_PAD_LEFT);
} else if(       $y       ){
  $options = str_pad($y,4,'0',STR_PAD_LEFT);
} else if(             $m ){
  $params .= '&nonefound';
}

if( strlen($options) > 0 ){
  $target_file = $options.'/index.html';
} else {
  $target_file = 'index.html';
}
if( !file_exists( $blog_path.$target_file ) ){
  $target_file = 'index.html';
  $params .= '&nonefound';
}
header( 'Location: '.$blog_url.$target_file.$params );

?>