Date

HV:

// Display mp3 five days after airdate
if ( strtotime( '-5 days' ) > strtotime( $airdate ) )
    $audio_url = 'http://hearvox.com/episode/cast/' . $airyr . '/' . $fileID . '.mp3';

<?php
$d = new DateTime( '2010-01-08' );
$d->modify( 'first day of next month' );
echo $d->format( 'F' ), "\n";
?>

http://www.php.net/manual/en/datetime.modify.php

http://www.php.net/manual/en/datetime.diff.php