"A function is a a self-contained block of code that can be called by your scripts. When called, the function's code is executed and performs a particiular task. You can pass values to a function, which then uses the values appropriately - storing them, transforming them, displaying them...When finished, a function can also pass a value back to the original code that called it into action." (132)
The abs() function is one of PHP's built-in functions, which are always available to any PHP script. This particular function "requires a signed numeric value and returns the absolute value of that number." (133). In this instance, the absolute value of -321 is 321, which the script prints to the page.
<?php
$num = -321;
$newnum = abs($num);
echo $newnum;
?>