php Random pass generator

<?php
/* RAndom Pass Generator by bl4ck-dz*/
echo "
<html>
<head><title>Random Pass Generator</title></head>
<style>
body {
    background-color:#ededed;
    display:block;
    text-align:center;
    font-family:'Tangerine', serif;
    font-size:19px;
    }
#rand  {
    border:1px solid #61555F;
    padding:5px;
    margin:20px;
    border-radius:10px;
}
#sub {
    border:1px dashed #8907AA;
    font-size:19px;
    padding:2px;
    margin:5px;
    border:radius:10px;
    cursor:pointer;
    }
#sub:active{
    background:#5E6966;
    color:yellow;
    }
</style>
<body>
<form method='post' action='' id='form'>
lenght of password :<br>
<input name='rand' type='text' id='rand'><br>
<input name='submit' type='submit' value='generate' id='sub'>
</form>";
class randgen{
    private $alpha = "aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ0123456789";
    public function __construct () {
    if ($_POST['submit']) {
        echo "The Password : ". $this->random($dz);
        }
    }
    public function random ($dz){
        for ($i =1 ;$i<=$dz;$i++) {
            $randgen = $this->alpha[rand (0,strlen($this->alpha))];
            echo $randgen;
            }
        }
    }
$pass = new randgen();
$pass->random($_POST['rand']);
echo "</body></html>";
?>