<?php
// Construimos el array "dias"
$dias = array(
	"Lunes" => true,
	"Martes" => true,
	"Miercoles" => false,
	"Jueves" => true,
	"Viernes" => true,
	"Sabado" => true,
	"Domingo" => false
);
// Imprimimos siempre y cuando sea true...
$arr[] = array_keys($dias,true);
if(count($arr[0]) == 0){
	echo "No se ha encontrado ningun dia";
}elseif(count($arr[0]) == 1){
	echo $arr[0][0];
}else{
		foreach($arr[0] as $imp){
			echo $imp;
			if($arr[0][count($arr[0])-2] == $imp){
				echo " y ";
			}else{
			if($arr[0][count($arr[0])-2] != $imp && $arr[0][count($arr[0])-1] != $imp){
				echo ", ";
				}
			}
		
		}
	}
?>