and welcome to the Drupal Dev!

why would you try something like this,

create a variable array that contain all the Hollidays>

$currentyear=the current year;
$holiday['01-01-'.$currentyear,other holiday,other holiday];

$result=$start_date->difference($end_date, 'days');
for(ctr=0;ctr=sizeof(holiday);ctr++;)
{
	if(holiday[ctr]>=start_date && holiday[ctr]<=end_date)
	temp++;

} 
$result -= temp;