how to apply condition for imprmir values ??of an array

hello staff www.ads-software.com
I need help from you again,

this function shuffle numbers,

function gen_num()
{
$caracteres = "012345678910111213141516171819";
$mistura = substr(str_shuffle($caracteres),0,15);
print $mistura;

I needed a condition to print or not the numbers at the posts, and the @leglesslizard User helped me. It works perfectly,

function gen_num()
{
global $post;
$mistura = get_post_meta( $post->ID, 'my_custom_meta', true );

if ( '' == $mistura ) {
$caracteres = "012345678910111213141516171819";
$mistura = substr(str_shuffle($caracteres),0,10);
update_post_meta( $post->ID, 'my_custom_meta', $mistura );
}

print $mistura;
}

I am now trying to make arrays and I can not!

function shuflenames()
{
$input = array("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
$rand_keys = array_rand($input, 2);
echo $input[$rand_keys[0]] . "\n";
echo $input[$rand_keys[1]] . "\n";

my failed attempt:

function shuflenames()
{
global $post;
$mixnames = get_post_meta( $post->ID, 'description', true );
if ( '' == $mixnames) {
$input = array("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
$rand_keys = array_rand($input, 2);
update_post_meta( $post->ID, 'description', $mixnames );
}
echo $input[$rand_keys[0]] . "\n";
echo $input[$rand_keys[1]] . "\n";
}

but the words change as the condition does not exist. every new access words are changed, they are not actually written in the posts

Thanks to all, Best Regards.