Little PHP help?

Hello. I am having this little problem , here is my code;

<?php
header("Location: msgchanger3.html");
$ourFileName = "test.txt";
$ourFileHandle = fopen($ourFileName, 'w') or die("can't open file");
$handle = fopen( "test.txt", "a");
$value = $_POST['111'];
$y = '1500';
$x = $value - $y; 
fwrite($handle, $x);
fclose($handle);
exit;
?>

PHP:

I need it to subtract from the "$y" and save the subtracted value so next time you subtract an input it subtracts from the last calculation. I don't know it that makes sense but if you understands please tell me how to , or an approach I should think about on how to do this. Thanks!


By The way. I have only been coding PHP for a week or so , so I understand the basics and I am just starting to get more complicated , so if you could keep it as noob friendly as possible , thank you

 

I would use a database table to hold the temporary value

 

<?php
// Get the value submitted via form
$value = $_POST['111'];
// Load file content of text.txt value
$con = file_get_contents('test.txt');
// Subtract posted value from saved value
$new_value = $con - $value;
// Save new value to text.txt file
file_put_contents('test.txt', $new_value);

// Redirect
header('Location: msgchanger3.html');
exit();

PHP:

Something like that?