Since you are using PDO one assumes you are using prepared statements so a function is not really going to suit. Now you can store those statements any number of ways; in an array, as simple string vars, or even as individual query classes.
Probably individual strings is the least complex solution and it requires less processing than using an associative array just to store the same query strings.
<?php // queries.php
$query1 = "SELECT col1, col2, col3 FROM table1 WHERE col1 = ?";
$query2 = "SELECT col1, col2 FROM table1 INNER JOIN table2 WHERE col1 = ? AND col2 = ?";
?>
<?php // page1.php
require_once('dbclass.php');
require_once('queries.php');
// connect etc
$params = array($_POST['id']);
$stmt = $db->prepare($query1);
$stmt->execute($params);
Now I personally a PDO prepared statement class that I use for this: just pass the query string and params array to the class and call the other methods to return results, rowcount etc.
