How about this?
$dow = date("w", mktime(0,0,0,4,1,2001));
gives you the day of week for April 1 2001. This is "0" for a Sunday and "6" for Saturday. So, if it's "0", you know that the first working day is 2nd April (one day further on). If it's "6", the first working day is 3rd April (two days further on).
Any other value means the 1st is a week day.