Description:
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I was wondering what is the default type of parameter when none is set? 

Reproduce code:
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If I run that code :

$dbh = new PDO(
  'mysql:host=localhost;dbname=xxx', 'xxx', 'xxx');
$stmt = $dbh->prepare('SELECT ? FROM DUAL WHERE 1 = ?');

var_dump($stmt->execute(array('aa\'aa', 'aaa')));

This dump true, meaning that 'aaa' was casted to a string and that the actual query looks like this in mySQL :

SELECT 'aa''aa' FROM DUAL WHERE 1 = 'aaa'

Expected result:
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The documentation should indicate that when no type is provided (while calling bindParam(), bindValue(), or execute()), PDO::PARAM_STRING is used. 

Actual result:
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Neither bindParam(), bindValue() or execute() speak of a default type, nor the main page of PDO (http://fr.php.net/manual/en/ref.pdo.php) describe it. Thus, we could expect anything from the driver (like casting variable into integer instead of string).