Why can't I assign any variable to a function return value in pChart?

I'm trying to assign a return value from a method to an array and in a second step to place this array into the addPoints() method of pChart.

Right now I'm using the following code:

$dataUsage = array();
for ($i = 1; $i < 31; $i++) {
        $dataUsage[$i - 1] = getDailyUsage($i);
}
$dataUsage[30] = getDailyUsage(31);

The getDailyUsage method looks like this:

function getPeriod() {
    return date("Y-m");
}

function getDailyUsage($day) {
        $var = getPeriod() . "-" . $day;
        global $pdo;
        $sql = "SELECT COUNT(  `id` ) FROM  `logs` WHERE timestamp = '$var'";
        $q = $pdo->prepare($sql);
        $q->execute();
        $q->setFetchMode(PDO::FETCH_ASSOC);
        $number = $q->fetchColumn(0);
        return $number;
}

To display the data into the pChart I'm running this:

$myData->addPoints($dataUsage, "Serie2");

As output I'm getting the default html icon as if I'm not finding the picture I'm looking for.

What really makes me curious is that I don't get any errors if I run this code:

$array = array();
for ($i = 1; $i < 31; $i++) {
    $array[$i - 1] = $i;
}
$array[30] = 31;

$myData->addPoints($array, "Absissa");

Since this is my first post I'm looking for tips to improve my questions, so feel free to leave a comment.