The new object oriented packages in PHP are your friend. Use DateTime: http://www.php.net/manual/en/class.datetime.php
<?php
$workDays = array("Thursday", "Saturday");
$d = new DateTime; //creates a datetime object, by default, the current time
while(!in_array($d->format("l"), $workDays)) $d->add(new DateInterval('P1D'));
print "Next work day: " . $d->format("Y-m-d");