public function doQuery ($aParameter) {
    .
    .
    .
    $sqlQuery = mysql_query("SELECT * FROM Table WHERE id = '".$aParameter."'");
    .
    .
    .
}

public function doQuery ($aParameter) {
    .
    .
    .
    $sqlQuery = mysql_query(THE_QUERY);
    .
    .
    .
}

define("THE_QUERY",     "SELECT * FROM Table WHERE id = '".$aParameter."'");

define("Q_QUERY1", "SELECT * FROM Table WHERE id = :1");
$cn = new PDO();
/* more code ... */
$arr = array();
$arr[":1"] = $parameterValue;

$s = $cn->prepare(Q_QUERY1);
$s->execute($arr);