Parse and use Lichess stream data with cURL and PHP

You should use CURLOPTWRITEFUNCTION instead of CURLOPTRETURNTRANSFER in order to fetch the stream data in time:

curlsetopt($ch, CURLOPTWRITEFUNCTION, function (\CurlHandle $ch, string $data) {
    # The tail of the stream data may not contain a complete line,
    # save the last element to this variable and prepend it to the next data.
    static $lastline = '';
    $len = strlen($data);
    if($len > 0) {
        $lines = explode("\n", $data);
        $lines[0] = $lastline.$lines[0];
        $linecount = count($lines) - 1;
        for($i = 0; $i < $linecount; $i++) {
            # handle each line here
            $j = json_decode($lines[$i]);
        }
        $lastline = $lines[$linecount];
    }
    return $len;
});