What is 'copy on write' behavior?

This is a memory management optimization in PHP (and other languages/systems), where copying a variable's value does not happen immediately, but only when attempting to modify the copy or the original if they refer to the same memory location. Before modification, both variables simply refer to the same data.

<?php
$a = 'string'; // $a points to a memory block with the string "string"
$b = $a; // $b now also points to the same memory block, no copying occurs

// Only now, when $b is modified, PHP creates a new copy of the data for $b
$b .= ' modified';

echo $a; // Outputs: string
echo $b; // Outputs: string modified
?>