Not sure how to progress this PHP problem

I'm trying to connect to an API using the code I was given for the purpose below. It fails on line 9 with the error:-

"Fatal error: Using $this when not in object context"

Can someone help me understand how to resolve this?

Thanks

function get_response($res) {
    $size_packed = fread($res, 4);
    if(strlen($size_packed) == 0) { return; }
    $size = unpack("N", $size_packed);
    // This works around PHP's fread() capping at 8192 bytes
    $out = "";
    $last = "";
    for($s = $size[1]-4; $s>0; $s-=strlen($last)) {
        $last = fread($this->res, $s);
        $out.=$last;
    }
    return $out;
}

$api_host = "api.domain.com";
$api_port = 1701;
$clID = "MyID";
$pw = "MyPassword";

$res = fsockopen("tls://" . $api_host, $api_port);

get_response($res);

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