如何将多个文件合并为一个

I am trying to combine all files into one txt file.However , i can't write texts into txt file . Here 's my code

public function combineFiles($filename,$source)
{
$files = glob("$source/*.??");
$out = fopen("$filename", "w");
foreach ($files as $file) {
        file_put_contents("$source/$filename.txt",$file);       
    }

return $filename.'.txt';

}