Find out the date the variable was modified?

Question

PHP

There is a code that changes the value of a variable every 3 days.

<?php

$var = 10000;
$increment = 500;

$day = date('j');
$var += $increment * floor(($day-1)/3); 
echo $var;
?>

How can I find out and display the current date of change of this variable?

Answer 1

Just write code that will update another variable (constant?) and update them at the same time.
Or by the time of the last modification of the file (if the write occurs only when the variable changes, and not with each access).
Actually, everything.