Arduino sketch code and PHP code for H-art tweeting weather station. This projects was tested with an Arduino UNO + ethernet shield + DHT22 sensor.

meteotweet.php

<?php

function roundNum($n) { return round(10 * $n)/10; }


// Insert your keys/tokens
$consumerKey 	= '<your-key>';
$consumerSecret = '<your-key>';
$oAuthToken 	= '<your-key>';
$oAuthSecret 	= '<your-key>';


// Full path to twitteroauth.php (change oauth to your own path)
require_once($_SERVER['DOCUMENT_ROOT'].'/oauth/twitteroauth.php');

// create new instance
$tweet = new TwitterOAuth($consumerKey, $consumerSecret, $oAuthToken, $oAuthSecret);

/* Create persistent DB connection */
$dbh = new PDO('mysql:host=localhost;dbname=arduino', 'root', '', array(
    PDO::ATTR_PERSISTENT => true
)); 

/* Insert values read from sensor DHT22 */
$stmt = $dbh->prepare("INSERT INTO dht22 (cdate, ctime, temperature, humidity) VALUES (:cd, :ct, :t, :h)");
$stmt->bindParam(':cd', $cd);
$stmt->bindParam(':ct', $ct);
$stmt->bindParam(':t', $t);
$stmt->bindParam(':h', $h);

$cd = date("Y-m-d");
$ct = date("H:i:s");
$t = $_POST['t'];
$h = $_POST['h'];

$stmt->execute();


/* Count rows inserted */
$collections = 0;
$stmt = $dbh->prepare("SELECT COUNT(id) FROM dht22");
if ($stmt->execute()) {
    $row = $stmt->fetch();
    $collections = (int)$row[0];
}

echo "{$collections} collections retrieved on database";


$collections_before_tweet = 750;

if (($collections % $collections_before_tweet) === 0) {


    $dt_tweet = date("M j, Y");
    $hr_tweet = date("H:i");
    $delay    = (int) $_POST['i']; // delay in seconds between sensor read
    $interval = (int) ($collections_before_tweet * $delay / 60); // interval minutes
    $interval_tweet = "{$interval}m.";

    if ($interval > 60) {
        $interval_tweet = floor($interval / 60) . "h." . $interval % 60 . "m.";
    }

    $avgquery  = "SELECT AVG(temperature), AVG(humidity) ";
    $avgquery .= "FROM dht22 ORDER BY cdate DESC, ctime DESC ";
    $avgquery .= "LIMIT {$collections_before_tweet}";
    $stmt = $dbh->prepare($avgquery);

    $avgt = 0;
    $avgh = 0;

    if ($stmt->execute()) {
        $row = $stmt->fetch();
        $avgt = roundNum(floatval($row[0]));
        $avgh = roundNum(floatval($row[1]));
    }



    $avgt = number_format($avgt, 1, '.', '');
    $avgh = number_format($avgh, 1, '.', '');

    // Your Message
    $message  = "On {$dt_tweet} at {$hr_tweet} ";
    $message .= "H-ART avg. temp. is {$avgt}C ";
    $message .= "and rel. humidity is {$avgh}% ";
    $message .= "(observing last ~{$interval_tweet}) ";
    $message .= "- via #arduino/EthShield #hartmakers";

    echo "\n$message";

    // Send tweet 
    $tweet->post('statuses/update', array('status' => "$message"));

stazione-meteo-hart-arduino.ino

#include <DHT22.h>
#include <SPI.h>
#include <Ethernet.h>
#include <LiquidCrystal.h>


// Data wire is plugged into port 6 on the Arduino
// Connect a 4.7K resistor between VCC and the data pin (strong pullup)
#define DHT22_PIN 6

int RSpin = 7;
int Enablepin = 8;
int D4pin = 9;
int D5pin = 3;
int D6pin = 4;
int D7pin = 5;

// Setup a DHT22 instance
DHT22 myDHT22(DHT22_PIN);


// Enter a MAC address for your controller below.
byte mac[] = { 0x90, 0xA2, 0xDA, 0x0F, 0x07, 0xFE };

IPAddress ip(192,168,0,2);
char server[] = "192.168.0.1";    
EthernetClient client;

boolean lastConnected = false;                    // state of the connection last time through the main loop
unsigned long lastConnectionTime = 0;             // last time you connected to the server, in milliseconds
const unsigned long postingInterval = 20 * 1000;  // delay between updates, in milliseconds

unsigned long lastSensorValues = 0;                 // state of the last sensor read time through the main loop
const unsigned long sensorInterval = 10 * 1000;     // delay between sensor read, in milliseconds


// initialize the library with the numbers of the interface pins  
LiquidCrystal lcd(RSpin, Enablepin, D4pin, D5pin, D6pin, D7pin);
  
String tmp = "";
String hum = "";




void setup(void) {

  
  // start serial port
  Serial.begin(9600);  

  lcd.begin(20, 4);
  lcd.print("H-art (Officina)");

  delay(3000);
  
}

void httpRequest(String t, String h) {

  Ethernet.begin(mac, ip);
  delay(1000);
  
  String query = "t=" + t + "&h=" + h + "&i=" + postingInterval/1000;

  if (client.connect(server, 80)) {
    Serial.println("connecting...");
    // send the HTTP PUT request:
    client.println("POST /oauth/meteotweet.php HTTP/1.1");
    client.println("Host: 192.168.0.1");
    client.println("Content-Type: application/x-www-form-urlencoded");
    client.println("Connection: close");
    client.println("User-Agent: arduino/1.0");
    client.print("Content-Length: ");
    client.println(query.length());
    client.println();
    client.println(query);
    client.println();

  }
  else {
    // if you couldn't make a connection:
    Serial.println("connection failed");
    Serial.println("disconnecting.");
    client.stop();
  }

  // note the time that the connection was made:
  lastConnectionTime = millis();

}

void getSensorValues(void) {
  
  String tmpString = "";
  String humString = "";

  DHT22_ERROR_t errorCode;
  
  errorCode = myDHT22.readData();
  switch(errorCode)
  {
    case DHT_ERROR_NONE:
      Serial.print("Got Data ");
      Serial.print(myDHT22.getTemperatureC());
      Serial.print("C ");
      Serial.print(myDHT22.getHumidity());
      Serial.println("%");
      // Alternately, with integer formatting which is clumsier but more compact to store and
      // can be compared reliably for equality:
       
      char buf[128]; 
      
      sprintf(buf, "Integer-only reading: Temperature %hi.%01hi C, Humidity %i.%01i %% RH",
                  myDHT22.getTemperatureCInt()/10, abs(myDHT22.getTemperatureCInt()%10),
                  myDHT22.getHumidityInt()/10, myDHT22.getHumidityInt()%10);
      Serial.println(buf);
      
   
      lcd.setCursor(0, 1);
      tmp = String(myDHT22.getTemperatureCInt()/10) + "." + String(abs(myDHT22.getTemperatureCInt()%10));
      tmpString = "Temp:     " + tmp  + " C";
      lcd.print(tmpString);
      
      lcd.setCursor(0, 2);
      hum = String(myDHT22.getHumidityInt()/10) + "." + String(myDHT22.getHumidityInt()%10);
      humString = "Humidity: " + hum + " %";     
      lcd.print(humString);
      
      break;
      
    case DHT_ERROR_CHECKSUM:
      Serial.print("check sum error ");
      Serial.print(myDHT22.getTemperatureC());
      Serial.print("C ");
      Serial.print(myDHT22.getHumidity());
      Serial.println("%");
      break;
      
    case DHT_BUS_HUNG:
      Serial.println("BUS Hung ");
      break;
      
    case DHT_ERROR_NOT_PRESENT:
      Serial.println("Not Present ");
      break;
      
    case DHT_ERROR_ACK_TOO_LONG:
      Serial.println("ACK time out ");
      break;
      
    case DHT_ERROR_SYNC_TIMEOUT:
      Serial.println("Sync Timeout ");
      break;
      
    case DHT_ERROR_DATA_TIMEOUT:
      Serial.println("Data Timeout ");
      break;
      
    case DHT_ERROR_TOOQUICK:
      Serial.println("Polled to quick ");
      break;
    }
    
    lastSensorValues = millis(); 
}


void loop(void) { 
  

   if (client.available()) {
       char c = client.read();
       Serial.print(c);
   }

   // if there's no net connection, but there was one last time
   // through the loop, then stop the client:
   if (!client.connected() && lastConnected) {
       Serial.println();
       Serial.println("disconnecting.");
       client.stop();
   }

  
   if(millis() - lastSensorValues > sensorInterval) {      
        getSensorValues();
   }
   

   // if you're not connected, and ten seconds have passed since
   // your last connection, then connect again and send data:
   if(!client.connected() && (millis() - lastConnectionTime > postingInterval)) {
       httpRequest(tmp, hum);
   }
   
   // store the state of the connection for next time through
   // the loop:
   lastConnected = client.connected();
}