How to write file in UTF-8 format?

Add BOM: UTF-8

file_put_contents($myFile, "\xEF\xBB\xBF".  $content); 

file_get_contents / file_put_contents will not magically convert encoding.

You have to convert the string explicitly; for example with iconv() or mb_convert_encoding().

Try this:

$data = file_get_contents($npath);
$data = mb_convert_encoding($data, 'UTF-8', 'OLD-ENCODING');
file_put_contents('tempfolder/'.$a, $data);

Or alternatively, with PHP's stream filters:

$fd = fopen($file, 'r');
stream_filter_append($fd, 'convert.iconv.UTF-8/OLD-ENCODING');
stream_copy_to_stream($fd, fopen($output, 'w'));

<?php
function writeUTF8File($filename,$content) { 
        $f=fopen($filename,"w"); 
        # Now UTF-8 - Add byte order mark 
        fwrite($f, pack("CCC",0xef,0xbb,0xbf)); 
        fwrite($f,$content); 
        fclose($f); 
} 
?>