Prepared statements with a variable number of parameters

I'm using PDO in PHP for database access (actually Zend Framework), and
following best practice by using parameters in prepared statements.

My question is, what do people normally do when there's a variable
number of parameters? - for example:

select ..... where item_id in ( ... );

In this case, I mightn't know how many values will go inside the
parentheses.

Thanks,

Ray.

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Raymond O'Donnell, Director of Music, Galway Cathedral, Ireland
rod(at)iol(dot)ie
Galway Cathedral Recitals: http://www.galwaycathedral.org/recitals
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