Example of MySQL FIND_IN_SET function

 FIND_IN_SET(str,strlist) returns a value in the range of 1 to N if the string str is in the string list strlist consisting of N substrings. A string list is a string composed of substrings separated by “,” characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic. Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function does not work properly if the first argument contains a comma (“,”) character.

mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2

In PHP:

$ids = array(1,2,3,4,5,6,7);
$stmt = $db->prepare( 'SELECT title FROM episode WHERE FIND_IN_SET(id,?)' );
$param = implode(',',$ids);
$stmt->bind_param('s',$param); 
$stmt->execute();

It can be used in the case that "in" clause can not be properly used in prepare statement. For example:

$stmt = $db->prepare( 'SELECT title FROM episode WHERE id in (?)');

is not working.

See also:

How to use variables in limit clause in MySQL stored procedure