How to Correctly Use PDO's IN Clause with Arrays: Positional vs. Named Placeholders?

Using PDO with IN Clause Array

When using PDO for database operations, utilizing arrays in the IN clause is commonly encountered. However, differences arise in the approach for positional placeholders versus named placeholders.

Positional Placeholders

For positional placeholders, the following code snippet demonstrates a correct approach:

$in_array = [1, 2, 3];
$in_values = implode(',', $in_array);
$my_result = $wbdb->prepare("SELECT * FROM my_table WHERE my_value IN ($in_values)");
$my_result->execute();
$my_results = $my_result->fetchAll();

However, the following code using named placeholders produces unexpected results:

$in_array = [1, 2, 3];
$in_values = implode(',', $in_array);
$my_result = $wbdb->prepare("SELECT * FROM my_table WHERE my_value IN (:in_values)");
$my_result->execute([':in_values' => $in_values]);
$my_results = $my_result->fetchAll();

This code would only return the item with the my_value equal to the first element in $in_array (1).

Solution

PDO handles these scenarios differently. To resolve this issue, dynamically create a string with placeholders and insert it into the query while binding array values as usual. For positional placeholders:

$in = str_repeat('?,', count($in_array) - 1) . '?';
$sql = "SELECT * FROM my_table WHERE my_value IN ($in)";
$stm = $db->prepare($sql);
$stm->execute($in_array);
$data = $stm->fetchAll();

For named placeholders, a more complex approach is required as you need to create a sequence of the named placeholders, e.g. :id0,:id1,:id2.

// other parameters that are going into query
$params = ["foo" => "foo", "bar" => "bar"];

$ids = [1,2,3];
$in = "";
$i = 0; 
foreach ($ids as $item)
{
    $key = ":id" . $i++;
    $in .= ($in ? "," : "") . $key; 
    $in_params[$key] = $item; 
}

$sql = "SELECT * FROM table WHERE foo=:foo AND id IN ($in) AND bar=:bar";
$stm = $db->prepare($sql);
$stm->execute(array_merge($params, $in_params)); // just merge two arrays
$data = $stm->fetchAll();

This approach allows for merging arrays in any order, unlike positional placeholders.

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