See Example #5 here.

/* Execute a prepared statement using an array of values for an IN clause */
$params = array(1, 21, 63, 171);
/* Create a string for the parameter placeholders filled to the number of params */
$place_holders = implode(',', array_fill(0, count($params), '?'));

/*
    This prepares the statement with enough unnamed placeholders for every value
    in our $params array. The values of the $params array are then bound to the
    placeholders in the prepared statement when the statement is executed.
    This is not the same thing as using PDOStatement::bindParam() since this
    requires a reference to the variable. PDOStatement::execute() only binds
    by value instead.
*/
$sth = $dbh->prepare("SELECT id, name FROM contacts WHERE id IN ($place_holders)");
$sth->execute($params);