In your specific case, instead of using placeholders, you could try building the SQL SELECT stmt based on the number of arguments present. For example,

$sql = "select clli, traffic_date, count(distinct(traffic_hour)) 
from gvc_traffic30 where 1=1 ";

$sql .= " and traffic_Date in ($date_ref_args) " if ( $date_ref_args )
+;
...etc...
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The "1=1" part enables you to add a dynamic number of "And " clauses without any problems...
hth