Force array to have all elements in string

$values_x = array();
for ($i = 0; $i < count( $series[0]['data'] ); $i++ ) {
    $values_x[] = $series[0]['data'][$i][0];
}

$values_y = array();
for ($i = 0; $i < count( $series[0]['data'] ); $i++ ) {
    $values_y[] = $series[0]['data'][$i][1];
}

$categories = array(
    "label"  => $label_x,
    "values" => $values_x,
    "colors" => $eco_chart_colors,
);

$series = array("label" => null, "values" => array( array("data" => $values_y)));

} else {

    $categories = array(
        "label"  => $label_x,
        "values" => $values_x,
        "colors" => "",
    );

    $series = array("label" => $label_y, "values" => $series);

}

The variable $value_x comes as numbers, ex: 2010, 2011, 2012, but I need to be strings ex: '2010', '2011', '2012'. Preferably I wanted to force everything that comes within $categories to be string, already tried with strval but always give me values: null .