over all you must set reading permission on your file, and than you can use the follow code:
$filename = "pclnumber.txt";
//read file content
$handle = fopen($filename, "r");
$number = fread($handle, sizeof($filename));
fclose($handle);
//update file content
$handleWrite = fopen($filename, "w+");
fwrite($handleWrite, $number+1);
fclose($handleWrite);
Bye, Marco