<?php

                            if(!empty($_POST[`submit`])){
                                    if(empty($_POST[`user_name`])||  
                                    empty($_POST[`phone_number`])||
                                    empty($_POST[`email`])||
                                    empty($_POST[`password`])|| 
                                    empty($_POST[`confirm_password`]))
                                {

                                    exit("please fill in all the fields.
                                        <a href = './register.php'>return</a>")
                                }

            if($_POST[`password`]!==$_POST[`confirm_password`])
            {   
                exit("passwords must match.
                    <a href = './register.php'>return</a>");
            }

        $pattern = "/^([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5})$/";
            if(!pregmatch($pattern,$POST[`email`]))
            {
               exit("please enter a valid email address.
                   <a href = './register.php'>return</a>");
            }

                    /*    $pwd = $_POST['password '];
                    if( strlen($password ) < 8 ) {
                    $error .= "Password too short!
                    ";
                    }
                    if( strlen($password ) > 20 ) {
                    $error .= "Password too long!
                    ";
                    }
                    if( strlen($password ) < 8 ) {
                    $error .= "Password too short!
                    ";
                    }
                    if( !preg_match("#[0-9]+#", $password ) ) {
                    $error .= "Password must include at least one number!
                    ";
                    }
                    if( !preg_match("#[a-z]+#", $password ) ) {
                    $error .= "Password must include at least one letter!
                    ";
                    if( !preg_match("#[A-Z]+#", $password ) ) {
                    $error .= "Password must include at least one CAPS!
                    ";
                    }
                    if( !preg_match("#\W+#", $password ) ) {
                    $error .= "Password must include at least one symbol!
                    ";
                    }
                    if($error){
                    echo "Password validation failure(your choise is weak): $error";
                    } else {
                    echo "Your password is strong.";
                    }*/

            $user_name     =addslashes($_POST[`user_name`]);
            $phone_number  =addslashes($_POST[`phone_number`]);
            $email         =addslashes($_POST[`email`]);
            $password      =addslashes($_POST[`password`]); 

        require_once("./connect.php");

        $sql="SELECT * FROM `customer_information_table` WHERE `email`='{email}'";  

        $result = $db->query($sql);
              if($db->error)
                {
                    exit("SQL error.
                         <a href = './register.php'>return</a>");
                }

              if($result->num_rows!==0)
              {
                exit("email address already exists.
                     <a href = './register.php'>return</a>");
              }

        //destroy new object and inserting data into db
         $result->free(); 
         $password=md5($password);
         $sql="INSERT INTO `customer_information_table`
               SET
                    `user_name`    ='($user_name)',  
                    `phone_number` ='($phone_number)',
                    `email`        ='($email)',
                    `password`     ='($password)'";

        $result = $db->query($sql);
           if($result === true)
           {
            echo"registration successful.<br/>";
           }
           else
           {
            echo"registration failed.<br/>";
           }

        $db->close();

        }

        ?> <!DOCTYPE html> <html> <title>Account information</title> <meta http-equiv='content-type' content = 'text/html';charset='utf-8'/> <script src="https://kit.fontawesome.com/1cc03fe4de.js" crossorigin="anonymous"></script><link rel="stylesheet" href="path/to/font-awesome/css/font-awesome.min.css"/> <style>
                *{margin:0px;padding:0px;}

                .header{
                text-align:center;
                }

                .fa-trash-alt
                {
                color:green;
                font-size:80px;
                }

                .footer {text-decoration:none;color:blue;text-align:center;}
                .separator{
                display:in-line block;width:30px;
                }

                .login
                {
                width:300px; height:400px;
                background:white;
                border:2px grey solid;
                margin:0 auto;
                -webkit-border-radius:5px;
                -mo-border-radius:5px;
                -o-border-radius:5px;
                -trident-border-radius:5px;
                padding:25px;
                }
                .fourth-level a{float:right;}

                .placeholder{width:1px;height:20px;}

                input{height:30px;width:300px;}

                </style> </head> <body> <div class='header'> <i class="fas fa-trash-alt fa-7x"></i> </div> <div class='login'> <div class='placeholder'></div> <h1 style="text-align: center;">Sign up</h1> <div class='placeholder'></div> <form action="" method="POST"> <label for="username">username </label> <input type="text" name="username" placeholder="enter your username ..."> <label for="phone number">phone number </label> <input type="phone number" name="phone number" placeholder="enter your phone number ..."> <label for="Email">Email </label> <input type="text" name="email" placeholder="enter your email ..."> <label for="password">password</label> <input type="password" name="password" placeholder="enter your password ..."> <label for="confirm password">confirm password </label> <input type="email" name="confirm_password" placeholder="confirm your password..."> <button name="submit" type="submit" value="submit">submit</button> </form> <span>if registered. <a href="signin.html">login </a> </span> </div> <!-- <div class='footer'> <div class='placeholder'></div> <div class='three-item-list'> <a href ='#'>conditions of use</a> <span class='separator'></span> <a href ='#'>privacy notice</a> <span class='separator'></span> <a href ='#'>help</a> </div> </div -->> </div> </body> </html>

You missed a semi-colon -

exit("please fill in all the fields.
                                        <a href = './register.php'>return</a>")

                                        //Change to --->
                                        exit("please fill in all the fields.
                                        <a href = './register.php'>return</a>");
commented: still not working properly in the web +0