准备语句，将字段作为参数奇怪的结果

i'm trying to parameterize the column in order to create a very dynamic query. What i tried is:

$db = new PDO("mysql:host=".$this->host.";dbname=myDB",$this->user,$this->pass);    
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$arr = array("g"=>"My Date","p"=>"Prono Score","a"=>"Age");
$param = "";
foreach($arr as $key=>$value) $param.=(':'.$key.',');
$param = substr($param, 0,count($param)-2);
$query = $db->prepare("SELECT $param FROM myTable LIMIT 1"); 
foreach($arr as $key=>$value) $query->bindValue($key,$value);
$query->execute();
$res = $query->fetchAll(PDO::FETCH_ASSOC);
return $res;

Having that results:

Array
(
    [0] => Array
        (
            [My Date] => My Date
            [Prono Score] => Prono Score
            [Age] => Age
        )

)

What is my error? i don't know why it happens.

With this line your binding placeholders called g, p, a to your select fields

$query = $db->prepare("SELECT $param FROM myTable LIMIT 1"); 

Then your assigning literal values to it with this line

foreach($arr as $key=>$value) $query->bindValue($key,$value);

Your really executing a query called

"Select 'My Date', 'Prono Score', 'Age' FROM myTable LIMIT 1"