Setting PDO/MySQL LIMIT with Named Placeholders

I'm having an issue binding the LIMIT part of an SQL query. This is because the query is being passed as a string. I've seen another Q here that deals with binding parameters, nothing that deals with Named Placeholders in an array.

Here's my code:

public function getLatestWork($numberOfSlides, $type = 0) {
 
 $params = array();
 $params["numberOfSlides"] = (int) trim($numberOfSlides);
 $params["type"] = $type;
 
 $STH = $this->_db->prepare("SELECT slideID 
     FROM slides
     WHERE visible = 'true'
         AND type = :type
     ORDER BY order
     LIMIT :numberOfSlides;");
 
 $STH->execute($params);
 
 $result = $STH->fetchAll(PDO::FETCH_COLUMN);
 
 return $result;        
 }
 

The error I'm getting is: Syntax error or access violation near ''20'' (20 is the value of $numberOfSlides).

How can I fix this?

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