Random letters from password check

Code:
&lt;?php

$password = 'password';

$required_letters = 3;

$letters = array();

while (count($letters) < $required_letters){

    $k = rand(1, strlen($password));

    $letters[$k] = substr($password, $k-1, 1);

}

ksort($letters);

if ($_POST['letter1']!=''){

    $p=1;

    for ($i=0; $i <= $required_letters; $i++) {

        if ($password[$_POST['key'.$i]-1]==$_POST['letter'.$i]){

            // nothing here

        }else{

            $p=0;    

        }

    }

    if ($p==1){

        echo 'Password Correct!';

    }else{

        echo 'Password Incorrect!';

    }

}else{

    echo '&lt;form name="input" action="test.php" method="post"&gt;';

    $i=1;

    foreach ($letters as $key => $value) {

        echo 'Please Type Letter '.$key.': ';

        echo '&lt;input type="hidden" name="key'.$i.'" value="'.$key.'"&gt;';

        echo '&lt;input type="password" name="letter'.$i.'" maxlength="1" style="width:25px"&gt;&lt;br>';

        $i++;

    }

    echo '&lt;input type="submit" name="submit" value="submit"&gt;';

    echo '&lt;/form&gt;';

}

?&gt;