We don't know the length of the sting
We only know how many chars to be removed from string.
Above code will remove last 3 chars from the string $q.
Some time we develop Query where we don't know how many items are there in the list. For example let us see this query.
SELECT * FROM `table_name` WHERE student_id IN ( 1,2,3,4)
To our original query we may add variable part like this
$query1="SELECT * FROM table_name WHERE student_id ";
$query2= " IN ( 1,2,3,4) ";
To generate this query we may have to loop through a for loop or any other loop and generate the variable part of the query string with one extra coma ( , ) at the end .
To remove the extra one char at the end we will use like this.