sql queries migrating to PHP7

I am now rewriting the old PHP5 scripts into PHP7, and I can’t figure out how to make the lookup of the words in the Database work. So I have two files db.php and dic_s.php

db.php
What I did here is just substituted mysql_ to mysqli_ and added a second paremeter $conn

<?php
  class db {

    function __construct()
    {
        global $dbh;
        if (!is_null($dbh)) return;
        $conn = mysqli_connect('localhost', 'user', 'password');
        mysqli_select_db($conn, 'dbname');
        mysqli_query($conn, 'SET NAMES utf8');
    }

    function select_list($query)
    {
        $q = mysqli_query($query);
        if (!$q) return null;
        $ret = array();
        while ($row = mysqli_fetch_assoc($q)) {
            array_push($ret, $row);
        }
        mysqli_free_result($q);
        return $ret;
    }
  }
?>

dic_s.php

//getting date thru $_POST
if (isset($_POST['search'])) {
// connection to database
  include('db.php');
   $db = new db();
     // filtering
       $word = trim(mysqli_real_escape_string($_POST['search']));

 // sql query
    $sql = "SELECT * FROM dictionary WHERE `word` LIKE '$word' GROUP BY `id` ORDER BY `word`";
  
     $row = $db->select_list($sql); 
      
    if(count($row)) {
        $end_result = ''; 
        foreach($row as $r) {
            
           $end_result     .= '<font color="lightblue" size="3" face="Arial">'.$r['word'] .
            '</font><i>('.$r['forms'] .')</i>';
      
           $end_result     .= '<br>Also:<i><font color="white" size="2" face="Arial"> '.$r['alterword'].'</i></font>';
           $end_result     .= '<br><ol><font color="#89F0F5" face="Arial" size="4"> '.$r['meaning'] .' </font>';
          }  echo $end_result;
} else {
        echo 'Nothing found.';
    }
}

I get these results while trying to execute the code

Warning: mysqli_real_escape_string() expects parameter 1 to be mysqli, object given in /dic_s.php on line 28

Warning: mysqli_query() expects at least 2 parameters, 1 given in /db.php on line 15

Nothing found.

For example, when I try to comply with errors and try to add the missing, it still won’t work

$q = mysqli_query($conn, $query);

Notice: Undefined variable: conn in /db.php on line 15

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in /db.php on line 15