PostHandler.php file

<?php
    //Get the contents of the file and put in a variable to get the length of the file.
    $read = file_get_contents("locations.txt");
    $fileLength = strlen($read);

    //Open file for editing ("w" stands for writing and will wipe the contents of a file)
    $myfile = fopen("locations.txt", "w");
    
    //Get contents from the post form submitted from C#.
    $name = $_POST["name"];
    $city = $_POST["city"];
    $lat = $_POST["lat"];
    $lng = $_POST["lng"];

    //Format the data into JSON so we can cast to a javascript object in the HTML page and write to file
    $serialized = ",{\"name\":\"" . $name . "\",\"city\":\"" . $city . "\",\"lat\":" . $lat .",\"lng\":" . $lng . "}";
    $newFileContents = substr_replace($read, $serialized, $fileLength - 1, 0);
    fwrite($myfile, $newFileContents);
    fclose($myfile);

    //Notify success
    echo 'Success';
?>