How to implement simple encryption technology in PHP

<?php
require_once &#39;config/database.config.php&#39;;

$act=$_REQUEST[&#39;act&#39;];
$username=$_POST[&#39;username&#39;];
$password=md5($_POST[&#39;password&#39;]);

if ($act==&#39;reg&#39;) {
  $sql="INSERT INTO user(username,password) VALUES(&#39;{$username}&#39;,&#39;{$password}&#39;)";
  $result=mysqli_query($link, $sql);
  
  if ($result) {
    echo "Success";
    echo "<meta http-equiv=&#39;refresh&#39; content=&#39;1;url=login.html&#39;/>";
  }else {
    echo "Failure!";
    echo "<meta http-equiv=&#39;refresh&#39; content=&#39;1;url=reg.html&#39;/>";
  }
}elseif ($act==&#39;login&#39;) {
  $sql="SELECT * FROM user WHERE username=&#39;{$username}&#39; AND password=&#39;{$password}&#39;";
  $result=mysqli_query($link, $sql);
  $validate=mysqli_fetch_array($result);
  //var_dump($validate);
  if ($validate) {
    echo "success";
    echo "<meta http-equiv=&#39;refresh&#39; content=&#39;1;url=http://www.baidu.com&#39;/>";
  }else {
    echo "failure";
    echo "<meta http-equiv=&#39;refresh&#39; content=&#39;1;url=login.html&#39;/>";
  }
}

mysql> SELECT * FROM user;
+----+----------+----------------------------------+
| id | username | password             |
+----+----------+----------------------------------+
| 1 | 123   | d41d8cd98f00b204e9800998ecf8427e |
| 2 | 123   | 5e12a8f9c9e959060fdcaea165393039 |
| 3 |     | d41d8cd98f00b204e9800998ecf8427e |
| 4 | root   | 202cb962ac59075b964b07152d234b70 |
| 5 | root   | 0c51f0ba4316a5c844397f69effe2d01 |
+----+----------+----------------------------------+

/**
 * Crypt
 */
echo crypt('shit');
echo "<hr/>";
echo crypt(&#39;shit&#39;,&#39;im&#39;);
echo "<hr/>";
if (CRYPT_EXT_DES) {
  echo crypt(&#39;shit&#39;,&#39;this is a test&#39;);
}
echo "<hr/>";
if (CRYPT_MD5) {
  echo crypt(&#39;shit&#39;,&#39;$1$this is a test$&#39;);
}

1223b8c30a347321299611f873b449ad
$1$ed0.Ph..$fPbfhSOMLyNdtZn9krT8X/
im37cLeO/JPaQ
th12A1V7QCns.
$1$this is $Bu9FE8Y8oGnIbftjDA4ez0

/**
 * Sha1
 */
echo "<hr/>";
echo sha1(&#39;shit&#39;);
echo "<hr/>";
echo sha1(&#39;shit&#39;,true);  
echo "<hr/>";
echo sha1(&#39;admin&#39;);

echo "<hr/>";
echo sha1(sha1(&#39;admin&#39;,true));
echo "<hr/>";
echo sha1(md5(&#39;admin&#39;));

/**
 * URL编码加密
 */

$str="this is a test";
$result=urlencode($str);
echo $result;
echo "<hr/>";
echo urldecode($result);
echo "<hr/>";
$str="login.php?username=shit&action=act%3 hape#123\\";
echo urlencode($str);
echo "<hr/>";

echo "<a href=&#39;index.php?username=shit&gender=male&#39;>Shit Login</a>";
print_r($_GET);
echo "<hr/>";
//所以地址栏进行一下编码，一方面是保密，一方面是处理特殊情况
//比如username&shit是一个整体，不编码的话，浏览器是看不出的
echo "<a href=&#39;index.php?username=username&shit&gender=male&#39;>Shit Login2</a>";
print_r($_GET);
echo "<hr/>";
$str="username&shit";
$str2="username=".urlencode($str)."&gender=".urlencode("male");
echo "<a href=&#39;index.php?".$str2."&#39;>Shit Login3</a>";
print_r($_GET);
echo "<hr/>";
//baidu example
//https://www.baidu.com/s?ie=utf8&wd=url%E7%BC%96%E7%A0%81%E5%8A%A0%E5%AF%86&tn=87048150_dg
//search url编码

this+is+a+test
this is a test
login.php%3Fusername%3Dshit%26action%3Dact%253+hape%23123%5C
Shit LoginArray ( [username] => username&shit [gender] => male ) 
Shit Login2Array ( [username] => username&shit [gender] => male ) 
Shit Login3Array ( [username] => username&shit [gender] => male )

/**
 * Base64
 */

$data="I am king";
echo base64_encode($data);
echo "<hr/>";
echo base64_decode(base64_encode($data));
echo "<hr/>";
echo base64_encode("中文测试");
echo "<hr/>";
// echo base64_decode("R0lGODlhAQABAIAAAP///wAAACH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==");
$data=file_get_contents("0.jpg");
echo base64_encode($data);