Laravel5 how to do a multi-condition query with if clause

I am a beginner, and currently creating a laravel project which needs multi-condition query like this:

view layer sample

Each of these three conditions could be blank.

If one is blank, the corresponding controller should not add 'where clause' when querying the db.

I know in java, I can apply if clause to this like:

String sql = "select field from table where 1 = 1";
if(a!=null&&!a.isEmpty())
  sql+=" and a = '"+a+"'";
then execute sql......

but in laravel, how do i address this?

via Chebli Mohamed