May this code help you. It works in my case.
$filename = "D:\xampp\htdocs\wordpress/wp-content/uploads/json/2018-10-25.json";
$fileUrl = "http://localhost/wordpress/wp-content/uploads/json/2018-10-25.json";
if(!file_exists($filename)):
$handle = fopen( $filename, 'a' ) or die( 'Cannot open file: ' . $fileUrl ); //implicitly creates file
fwrite( $handle, json_encode(array()));
fclose( $handle );
endif;
$response = file_get_contents($filename);
$tempArray = json_decode($response);
if(!empty($tempArray)):
$count = count($tempArray) + 1;
else:
$count = 1;
endif;
$tempArray[] = array_merge(array("sn." => $count), $data);
$jsonData = json_encode($tempArray);
file_put_contents($filename, $jsonData);