Can't manage to make $_POST to work - php

Question

I have three files in a server (000webhost.com):
"Test01.php" (main file),
"database.txt" (saved data, which will be changed by users),
"save_txt.php" (the file which gets data from the main file and writes it to the "database".
"Test01.php" is supposed to show a simple list, with a few names in a table (single column, multiple lines).
Those names will be retrieved from a file named "database.txt".
Everytime some user click on a name, that name will be sent to the bottom of the list, and the list will be saved to "database.txt", so the next user will see the changes made by the last one.
A function in "Test01.php" sends the changed list to a second file ("save_txt.php"), which is supposed to write it back to "database.txt".
I can manage to retrieve the data from the txt file, and the clicking events as well, but I still can't find a way to save the data into the txt file...
In fact, I don't understand why my variable isn't seen from inside the second php file ("save_txt.php").
To retrieve data I use:
<?php
$Data_from_File = file("database.txt",FILE_IGNORE_NEW_LINES);
?>
And the script:
var sSaved_Data = <?php echo json_encode($Data_from_File); ?>;
The listing stuff works fine.
I get many names from the txt file and store it into an array. Then I display it in the table. No problem from reading the file.
I send data to "save_txt.php" by doing this:
var sNew_Data = " is blue";
.
.
.
xmlhttp.send("php_Data_to_Save=" + sNew_Data);
But, could anyone tell me why the simple code below doesn't work?
https://rbonphp.000webhostapp.com/Test01.php
"save_txt.php" is just like this:
<?php
$var1 = $_POST["php_Data_to_Save"];
echo $var1;
?>
In time: in this example I just want to see " is blue" echoed in the screen (no matter where). I just want to understand how to get the data back to "save_txt.php".
Later I will try to write $var1 to "database.txt".
But first things first...
:-(
As I said, this "Test01.php" is just a test. The list and all the clicking events I wrote in another file. That part works just fine.
* Edited *
Let's try to put it all in a few lines.
The main file (Test01.php) does:
var sNew_Data = " is blue";
// there's more code for the XMLHttpRequest function
xmlhttp.send("php_Data_to_Save=" + sNew_Data);
The secondary file (save_txt.php) does:
<?php
$var1 = $_POST["php_Data_to_Save"];
echo $var1;
?> // and this is ALL its code, just these 4 lines.
That line echo $var1; should simply show " is blue" on the screen.
Right???
Extra info: Test01.php is a step prior to make the following page to work:
https://rbonphp.000webhostapp.com/DailyTasks1.php

Answer 1

Your code is working.
Calling Send_Data_to_Server() returns "The sky is blue" which is what you wanted, as the sent data is var sNew_Data = sSaved_Data + " is blue"; and it is correctly echoed by save_txt.php
Note: your commented jquery ajax call is wrong however, that's not how you define the sent data, check the first example at http://api.jquery.com/jquery.ajax/
You'd write this:
$.ajax({
url: 'save_txt.php',
data: { php_Data_to_Save : sNew_Data },
type: 'POST'
});

Answer 2

first, you didnt call Send_Data_to_Server() anywhere, you've just declared it.
secont, you just send the request to server, but don't store the answer anywhere.