php paging code learning examples to share

<?php 
    header("content-type:text/html;charset=utf-8");
    // Database connection 
    $conn = mysql_connect("localhost", "root", "111") or die("not connnected : ".mysql_error());
    mysql_select_db("test", $conn);
    mysql_query("set names utf8");
    // Query how many rows of data there are 
    $sql1 = "select count(*) from user";
    $ret1 = mysql_query($sql1);
    $row1 = mysql_fetch_row($ret1);
    $tot = $row1[0]; 
    // Number of rows per page 
    $length = 5;      
    // Total number of pages   
    $totpage = ceil($tot / $length);
    // The current number of pages 
    $page = @$_GET['p'] ? $_GET['p'] : 1;
    //limit  The lower limit 
    $offset = ($page - 1) * $length;
    echo "<center>";
    echo "<h2>php padding</h2>";
    echo "<table width='700px' border='1px' >";
    echo "<tr>";
    echo "<th>ID</th>";
    echo "<th>USER</th>";
    echo "<th>PASS</th>";
    echo "</tr>";
    // Display the query data in a table 
    $sql2 = "select * from user order by id limit {$offset}, {$length}";
    $ret2 = mysql_query($sql2);
    while ($row2 = mysql_fetch_assoc($ret2)) {
        echo "<tr>";
        echo "<td>{$row2['id']}</td><td>{$row2['name']}</td><td>{$row2['pass']}</td>";
        echo "</tr>";
    }
    echo "</table>";
    // on 1 Under the page and 1 page 
    $prevpage = $page - 1;
    if ($page >= $totpage) {
        $nextpage = $totpage;
    } else {
        $nextpage = $page + 1;
    }
    // jump 
    echo "<h3><a href='index.php?p={$prevpage}'> on 1 page </a>|<a href='index.php?p={$nextpage}'> Under the 1 page </a></h3>";
    echo "</center>";

Emphasis:

< 1 > $sql2 = "select * from user order by id limit {$offset}, {$length}"; , $offset, $length, and page number.

< 2 > How to get the previous page and the next page, and the critical point.