Using compress cached file by ID

<?php    
    $file = "CACHE_DIR/$ID";
    if (!file_exists("CACHE_DIR/$ID")) {
        echo "<b>Load from server</b><br>\n";
        $source = file_get_contents($url);
        if ($source != "") {//cache
            $f = fopen ($file, 'w');
            fwrite($f, gzcompress($source, 9));
            fclose($f);
        }
    } else {
        $f = fopen ($file, 'r');
        $source = gzuncompress(fread($f, filesize($file)));
        fclose($f);
        echo "<b>Load from cached file: <i>CACHE_DIR/$ID</i></b><br>\n";
    }
    
    // do something with source
?>