functions.php

<?php

$dia = array("Domingo", "Lunes", "Mártes", "Miercoles", "Jueves", "Viernes", "Sábado");
$mes = array("", "Enero", "Febrero", "Marzo", "Abril", "Mayo", "Junio", "Julio", "Agosto", "Septiembre", "Octubre", "Noviembre", "Diciembre");

$actual_m = date("n");
$actual_d = date("j");
$actual_y = date("Y");

if ($timestamp = strtotime($_GET['d'])){
	$days = date('N', $timestamp);
	$days_number = date('t', $timestamp);

    $month = $mes[date('n', $timestamp)];

	$year = date('Y', $timestamp);
}

function PrimerDiaMes($mes,$año) {
    $date = strtotime($año."-".$mes."-01");

    $array_date = getdate($date);
    $year = $array_date["year"];
    $month = $array_date["mon"];
    $month_day = $array_date["mday"];

    $array_date = getdate(mktime(0, 0, 0, $month, 1, $year));
    return $primerDia = $array_date["weekday"];
}

?>