Deprecated features in php 7.1

The script bellow is working properly under php 5.6, but not in php 7.1. 
What could we do to fix it?

<?php

include("_restrito/conexao.php");

 $conexao = mysql_connect($host,$user,$pass,$db);

    if(!$conexao)

    die("Nao foi possivel conectar no servidor MySQL. Erro: " . 
mysql_error());

    mysql_select_db($db) or die("Nao foi possivel usar o banco de dados. 
Erro: " . mysql_error());

    $pagina = $_GET['pagina'];

    $banco = mysql_select_db("mensagem");

 $busca = "SELECT * FROM portfolio ORDER BY id DESC";

 $total_reg = "5";
 if (!$pagina) {
    $pc = "1";
} else {
    $pc = $pagina;
}

$inicio = $pc - 1;
$inicio = $inicio * $total_reg;

$limite = mysql_query("$busca LIMIT $inicio,$total_reg");
$todos = mysql_query("$busca");

$tr = mysql_num_rows($todos);
$tp = $tr / $total_reg;
while ($dados = mysql_fetch_array($limite)) {

?>

Thanks in advance for any help.
Att.,
Paulo