How to return an array from a non-native function call? [duplicate]

I am getting an error on this line:

$ret = array_merge($ret, preg_ls($path . "/" . $e, $rec, $pat));

Error is:

array_merge() Argument #2 is not an array

I don't know know to solve this.

function preg_ls($path = ".", $rec = false, $pat = "/.*/") {
    // it's going to be used repeatedly, ensure we compile it for speed.
    $pat = preg_replace("|(/.*/[^S]*)|s", "\\1S", $pat);
    //echo($pat);
    //Remove trailing slashes from path
    while (substr($path, -1, 1) == "/")
        $path = substr($path, 0, -1);
    //also, make sure that $path is a directory and repair any screwups
    if (!is_dir($path)) $path = dirname($path);
    //assert either truth or falsehoold of $rec, allow no scalars to mean truth
    if ($rec !== true)
        $rec = false;
    //get a directory handle
    $d = dir($path);
    //initialise the output array
    $ret = Array();
    //loop, reading until there's no more to read
    while (false !== ($e = $d->read())) {
        //Ignore parent- and self-links
        if (($e == ".") || ($e == "..")) continue;
        //If we're working recursively and it's a directory, grab and merge
        if ($rec && is_dir($path . "/" . $e)) {
            $ret = array_merge($ret, preg_ls($path . "/" . $e, $rec, $pat));
            continue;
        }
        //If it don't match, exclude it
        if (!preg_match($pat, $e))
            continue;
        //In all other cases, add it to the output array
        //echo ($path . "/" . $e . "<br/>");
        $ret[] = $path . "/" . $e;
    }
    //finally, return the array
    echo json_encode($ret);
}