(OTP-Auth) Easily Create One-Time Password System in PHP App

otp-auth.php

<?php
/**
 * Name: (OTP-Auth) One-Time Password Authentication
 * Auth: Jason Jersey
 * Date: 12-24-2017
 * Link: https://gist.github.com/icryptix/130fa81ca15925b8553bec8700e5813d
 */

/* Verifying whether OTP 1 hour cookie is set */
if(isset($_COOKIE["sitename_otp_cookie"])){
    /* OTP Passthrough */
    /* Display content to users who enter correct OTP code */
    echo "HTML GOES HERE";
} else {
    /* No Cookie Set */
    /* Get Current Page URL */
    $REQT_URI = $_SERVER['REQUEST_URI'];
    /* Open Previously Started Session */
    session_start();

    /* Session Is Set */
    if(isset($_SESSION['sitename_otp_user'])) {

		if(isset($_POST['submitOTPcode']) && !empty($_POST['submitOTPcode'])){
		    if($_SESSION['sitename_otp_user'] == $_POST['submitOTPcode']) {
			    $cookie_value = $_POST['submitOTPcode'];
			    setcookie('sitename_otp_cookie',$cookie_value, time() + 3600, '/');
			    /* Refresh page */
			    header("Location: $REQT_URI");
		    } else {
		        echo "<div class='otp-error'>Incorrect OTP code entered. Try again!</div>";
		    }
		}
		echo "<div class='otp-code'>We've just emailed you a code to login. Please check your email now and enter the code below to safely access this site. This is an additional security messaure we take to protect your account.</div>";
		echo "<form method='post'>";
		echo "<input type='text' name='submitOTPcode' id='submitOTPcode' value='' placeholder='00000' size='5' maxlength='5'>";
		echo "<input type='submit' value='Verify OTP'>";
		echo "</form>";

    } else {

        /* Session Not Set */
        $otpstr = '';
        for($i=5;$i>0;$i--){
            $otpstr = mt_rand(0,99999);
        }
        /* Prepare Email */
        $from = '[email protected]'; // Sent from the site
        $to = '[email protected]'; // Sent to the user
        $subject = 'One-time Password for Login';
        $body = 'Your one-time password is: '.$otpstr;
        $headers .= 'From: '.$from. "\r\n" .
                'Reply-To: '.$from. "\r\n";
        /* Send Email */
        mail($to, $subject, $body, $headers);

        /* Start New Session */
        session_start();
        $_SESSION['sitename_otp_user'] = $otpstr;
        /* Refresh page */
        header("Location: $REQT_URI");
    }
}