"; // Trying to change a constant will give an error: // MAX_WIDTH += 1; // But changing a variable will not. $max_width += 1; echo $max_width; /* Note that once a page is returned, a constant CAN be redefined by another PHP page. For example: Browser Request 1 -> page1.php -> SIZE defined as 10 -> PHP page finishes -> Page 1 Returned Browser Request 2 -> page2.php -> SIZE defined as 20 -> PHP page finishes -> Page 2 Returned SIZE must remain 10 throughout page1.php, but when the 2nd request comes in SIZE is not defined */ ?>