PHP fatal error

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1

My website: https://my.hosteyme.ga/login.php

View error by login: Username: hmp_19060100 , Password: ABc123

Error:

Fatal error: Uncaught Error: Call to undefined function session_register() in /home/vol3_2/unaux.com/unaux_23970071/my.hosteyme.ga/htdocs/login.php:21 Stack trace: #0 {main} thrown in /home/vol3_2/unaux.com/unaux_23970071/my.hosteyme.ga/htdocs/login.php on line 21

login.php:

     `<?php
       include(config.php);
       session_start();
       
       if($_SERVER[REQUEST_METHOD] == POST) {
          // username and password sent from form 
          
          $myusername = mysqli_real_escape_string($db,$_POST['username']);
          $mypassword = mysqli_real_escape_string($db,$_POST['password']); 
          
          $sql = SELECT * FROM hmp_account WHERE username = '$myusername' and password = '$mypassword';
          $result = mysqli_query($db,$sql);
          $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
          $active = $row['active'];
          
          $count = mysqli_num_rows($result);
          
          // If result matched $myusername and $mypassword, table row must be 1 row
    		
          if($count == 1) {
             session_register(myusername);
             $_SESSION['login_user'] = $myusername;
             
             header(location: welcome.php);
          }else {
             $error = Your Login Name or Password is invalid;
          }
       }
    ?>
    <html>
       
       <head>
          <title>HMP Login</title>
          <link rel=shortcut icon href=image/favicon.ico type=image/x-icon>
          
          <style type = text/css>
             body {
                font-family:Arial, Helvetica, sans-serif;
                font-size:14px;
             }
             label {
                font-weight:bold;
                width:100px;
                font-size:14px;
             }
             .box {
                border:#666666 solid 1px;
             }
          </style>
          
    <script async src=//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js></script>
    <script>
      (adsbygoogle = window.adsbygoogle || []).push({
        google_ad_client: ca-pub-7074728236863065,
        enable_page_level_ads: true
      });
    </script>
       </head>
       
       <body bgcolor = #66e0ff>
    
          <div align = center>
             <div style = width:300px; border: solid 1px #333333;  align = center>
                <div style = background-color:#333333; color:#FFFFFF; padding:3px;><img src=image/hmp.jpg alt=HMP Logo>
    	<b><h1>HMP Login</h1></b></div>
    				
                <div style = margin:30px>
                   
                   <form action =  method = post>
                      <label>Username  :</label><input type = text name = username class = box/><br /><br />
                      <label>Password  :</label><input type = password name = password class = box /><br/><br />
                      <input type = submit value =  Login /><br />
                   </form>
                   
                   <a href=https://help.hosteyme.ga>Lost Password</a>
    
                   <div style = font-size:11px; color:#cc0000; margin-top:10px><?php echo $error; ?></div>
    					
                </div>
    				
             </div>
    			
          </div>
    
    <script async src=//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js></script>
    <script>
      (adsbygoogle = window.adsbygoogle || []).push({
        google_ad_client: ca-pub-7074728236863065,
        enable_page_level_ads: true
      });
    </script>
       </body>
    </html>`

Please help! Thanks!