Try something like this to see if it works:
$file = fopen("1stock.js", "c+");
$number = fseek($file, 61, SEEK_CUR);
if($number == -1) {
throw new Exception("fseek() failed!"); // or however else you want to handle this error
}
$fh = fgetc($file); // fgetc() needs a file handle, not the return code from fseek()
print("DEBUG: Got this value from the file: $fh"); // or error_log(), or however you like to get debug info
fwrite($file, $fh -1);
Also, I'm not sure if the fseek() position is zeroth based or not. If it is, then character 61 would be offset 60.