<?php
$num = 21;

$r = array();
for ($i = 0; $i <= $num; $i++) {
    $r[] = rand(1,$num);
}

$newArray = array();
for ($i = 0; $i < 15; $i++) {
    $randVal = $r[rand(1, $num)];
        if (!in_array($randVal, $newArray))
            array_push($newArray, $randVal);
        else
    $i--;
}

foreach($newArray as $id=>$val){
    echo($id . '. ' . $val . '<br>');
}
?>