You can do the select UNIX_TIMESTAMP as Mordecai said above, or if you prefer to do the conversion in PHP instead of MySQL, you can do it this way.
Sub the $beforetime below for your value from the database...
$beforetime = "20:00:00";
echo "Before Conversion: ".$beforetime."<BR />";
// Make it into a Unix TimeStamp
$convertingtime = strtotime($beforetime);
// Convert it to the format you desire
$endtime = date("g:i:s A", $convertingtime);
echo "After Conversion: ".$endtime;