Add a variable in functions.php and call it in single.php

either use global variable
in functions.php:

    global $prevPost;
    $prevPost = 10;

in single.php :

    global $prevPost;
    if($prevPost)
     .. . . . Your code continues. .

Second method: define a function in functions.php and call that in single.php
in functions.php :

    function func_prevPost(){
     $prevPost = 10;
     return $prevPost;
    }

in single.php :

    if(func_prevPost()){
    // your code here
    }