How can i know if someone has opened an email?

There are many possibilities, which is unfortunate from a client's point of view, because many of them are abused by spammers who want to know whether their mail addresses are "alive".

The most exhaustive list of techniques is listed (and tested) by https://www.emailprivacytester.com

I think from your point of view, the most "kind" solution are the disposition notification headers. Asking for a "reliable" solution, however, opposes the general interest of the recipients that there should not be a way to reliably observe their behavior against their will.

Here is link to complete guide

[ [ Prerequisite: ] ] 1. You need download PHPMailerAutoload.php. 2. Please check your php version. 3. Please check your shared hosting allows phpmailer.

[ [ Privacy Policy ] ] 1. A recipient mail service provider obviously pops up a privacy alert when the recipient opens your email. So if he or she rejects your email, the desirable information could not be gained.

Step 1. Create a PHP web page with name “index.php”. This will serve as our front-end.We’ll send an email and track it from here only.

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Email Open Tracking Using PHP</title>
        <script src="js/jquery.min.js" type="text/javascript"></script>              
     <script src="js/myjs.js" type="text/javascript"></script>
        <link href="css/style.css" rel="stylesheet" type="text/css"/>

    </head> 
    <body>        
        <div id="main">
            <h1>Email Open Tracking Using PHP</h1>
            <div id="login">
                <h2>Send Email</h2>
                <hr/>
                <form id="form1"  method="post">
                    <div id="box">
                        <input type="email" placeholder="To : Email Id " name="mailto" required/>  
                        <input type="text" placeholder="Subject : " name="subject" required/>
                        <textarea rows="2" cols="50" placeholder="Meassage : This is the fixed message of test email to get notify when it is read...." name="message" readonly ></textarea>
                        <input type="submit" value="Send" name="send" />   
                    </div>                                     
                </form>

                <div id="loading-image"><img src="http://www.arabianbusiness.com/skins/ab.main/gfx/loading_spinner.gif" alt="Sending....."/></div>

                <form id="form2"  method="post">   
                        <div id="view"></div><br><br>
                    <div id="readstatus"></div>
                    <input type="submit" value="Track Status" id="track_mail" name="track"/>                           
                </form>
            </div>
        </div>
    </body>
</html>

Step 2. Create a PHP file with name “tracker.php”. There will be our PHP script for two purpose:

a. Send mail using PHP mailer library.

b. Read the log file(email.txt) to track if the email opened.

<?php

require ('phpmailer/PHPMailerAutoload.php');
$from = "test@gmail.com";    //sender's username
$pwd = "test@1234";         //sender's password
//-------------------------------------------------------SEND eMail----------------------------------------------------------------------
if (isset($_POST['mailto'])) {
    try {
        $mail = new PHPMailer(true); //New instance,exceptions enabled with true
        $to = $_POST['mailto'];
        $subject = $_POST['subject'];
        $id = rand(111, 999);
        $id.=rand(111, 999);
        $body = "This is the fixed message of test email to get notify when it is read.....";
        $body .= "<img border='0' src='https://yourwebsite.com/trackonline.php?email=$to&id=$id&subject=$subject' width='1' height='1' alt='image for email' >";
        $mail->IsSMTP();                           // tell the class to use SMTP
        $mail->SMTPAuth = true;                  // enable SMTP authentication
        $mail->Port = 25;                    // set the SMTP server port
        $mail->Host = "smtp.gmail.com"; // SMTP server
        $mail->Username = $from;     // SMTP server username
        $mail->Password = $pwd;            // SMTP server password
        $mail->From = $from;
        $mail->FromName = "TESTUSER";
        $mail->AddAddress($to);
        $mail->Subject = $subject;
        $mail->AltBody = "Please return read receipt to me."; // optional, comment out and test
        $mail->WordWrap = 80; // set word wrap
        $mail->MsgHTML($body);
        $mail->IsHTML(true); // send as HTML
        $mail->Send();

//return foll
        echo '<input id="id1" name="id" type="hidden" value="' . $id . '">'   
        . '<input id="email1" name="email" type="hidden" value="' . $to . '">'
        . '<label id="label1">Mail sent to <b>' . $to . '<b></label>';
    } catch (phpmailerException $e) {
        echo $e->errorMessage();
    }
}
////------------------------------------------READ email.txt-------------------------------------------------------

if (!empty($_POST['id'])) {

    $id = $_POST['id'];
    $to = $_POST['email'];
    $checkid = "Id:" . $id;
    $fh = fopen("https://yourwebsite.com/email.txt", "r");
    $read = false; // init as false
    while (($buffer = fgets($fh)) !== false) {
        if (strpos($buffer, $checkid) !== false) {
            $a = explode("%",$buffer);
            $read = true;
            break; // Once you find the string, you should break out the loop.
        }
    }
    fclose($fh);

    if ($read == true) {
        //$string = $email . " seen the mail on subject: '" . $sub . "' from ip: " . $ipAddress . " on " . $date . " and Id:" . $id . "\n";
        echo "<img id=\"closed-image\" src=\"img/envelope-open.png\" alt=\"email not opened\"/><br><p id=\"closed-para\">"
        . "Mail sent from <b>" . $from . "</b><br> To <b>" . $to
        . "</b><br>has been<div id=\"color-read\"> opened on <b>".$a[1]."</b></div></p>"
        . "<input id=\"id1\" name=\"id\" type=\"hidden\" value=\"" . $id . "\">";  //appended hidden input to keep previous data on the page.


    } else {
        echo "<img id=\"closed-image\" src=\"img/envelope-closed.png\" alt=\"email not opened\"/><br><p id=\"closed-para\">"
        . "Mail sent from <b>" . $from . "</b><br> To <b>" . $to
        . "</b><br><div id=\"color-not-read\"> Not yet opened......</div></p>"
        . "<input id=\"id1\" name=\"id\" type=\"hidden\" value=\"" . $id . "\">";  //appended hidden input to keep previous data on the page.
    }
}

Step 3. Create a PHP file with name “trackonline.php”. This will be the PHP script to log an entry in a text file also check for the duplicity.

<?php

if (!empty($_GET['email'])) {
    $id = $_GET['id'];
    $checkid = "Id:" . $id;
    $email = $_GET['email'];
    $sub = $_GET['subject'];
    date_default_timezone_set('Asia/Kolkata');
    $date = date('d/m/Y h:i:s a');
    $fh = fopen("email.txt", "a+"); //file handler
    $a = fgets($fh);
    $found = false; // init as false
    while (($buffer = fgets($fh)) !== false) {
        if (strpos($buffer, $checkid) !== false) {
            $found = true;
            break; // Once you find the string, you should break out the loop.
        }
    }
    if ($found == false) {
        $string = $email."opened the mail with subject:".$sub."on%".$date."% with mailId:".$id."\n";
        fwrite($fh, $string);
    }
    fclose($fh);

    //Get the http URI to the image
    $graphic_http = 'https://yourwebsite.com/blank.gif';

    //Get the filesize of the image for headers
    $filesize = filesize('blank.gif');

    //Now actually output the image requested, while disregarding if the database was affected
    header('Pragma: public');
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Cache-Control: private', false);
    header('Content-Disposition: attachment; filename="blank.gif"');
    header('Content-Transfer-Encoding: binary');
    header('Content-Length: ' . $filesize);
    readfile($graphic_http);

    //All done, get out!
    exit;
}