array rand pick one or more random entries

I have this code it's choose random name from the comments.txt file

<?php $f_contents = file("comments.txt");
$line = $f_contents[array_rand($f_contents)];
$messages = $line;  ?>

I need to make it choose from this data below

$messages = $_POST ['0'];
$messages2 = $_POST ['1'];
$messages3 = $_POST ['2'];

example:

<?php 
    $f_contents = $messages,$messages2,$messages3;
    $line = $f_contents[array_rand($f_contents)];
    $messages = $line;  
                        ?>