Boolian check giving incorrect answer

I'm trying to make a reputation system:

if($upvotetruedownvotefalse == false){
        if(strpos($lines[2],$username.",") !== false){
            echo "you've already downvoted";
        }
        else{
            echo "you haven't downvoted yet";
                $lines[2] = $username.",".$lines[2];
                $lines[0] = $lines[0]-1 . "\n";
                file_put_contents( $filename , $lines);

        }
    }
        if($upvotetruedownvotefalse == true){
        if(strpos($lines[1],$username.",") !== false){
            echo "you've already upvoted";
        }else{
            echo "you haven't upvoted yet";
                $lines[1] = $username.",".$lines[1];
                $lines[0] = $lines[0]+1 . "\n";
                file_put_contents( $filename , $lines);
        }

    }

The value of $upvotetruedownvotefalse is determined through AJAX

However these comparison operators is giving me wrong results.

For example if $upvotetruedownvotefalse = false then $upvotetruedownvotefalse == truewould be triggered. I've tested this by echoing the value of $upvotetruedownvotefalseand recieving the echo results (including the ones in the if statements)using a success function:

echo $upvotetruedownvotefalse;
success: function(response){
          alert(response);
}

Then I console.log the result and i would get this:

Falseyou haven't upvoted yet

I'm getting conflicting results. I've also tried using different types of === !=== in different places and I get either an inverted result or my success function fails

Is there something that I don't know about Boolean operators? Any help would be greatly apreciated