21. sort()和rsort()

Another common thing to do with arrays is sort them. Handily enough, PHP has a sort() function for just such an occasion!

$array = array(5, 3, 7, 1);
sort($array);
print join(", ", $array);
// prints "1, 3, 5, 7"

PHP also has the opposite function:rsort().

$array = array(5, 3, 7 ,1);
rsort($array);
print join(":", $array);
// prints "7:5:3:1"

Lastly, we use join(glue, array) so we can easily print out the representations of our sorted arrays in this exercise.

For this exercise, we'll use a comma (,) as the glue.