I noticed in the example on page 33 (boj-random-blog-posts.php below) that the function boj_randomly_order_blog_posts is called BEFORE it is defined. The function is defined immediately after the line that invokes it in the add_action hook. I am new to PHP (obviously ), but I do know that in other languages the function must be defined before they are called.

Is this allowed in PHP? If so, what is the best practice regarding this? Is it better to go one way or the other?

Example from book:


<?php

add_action( 'pre_get_posts', 'boj_randomly_order_blog_posts' );

function boj_randomly_order_blog_posts( $query ) {

if ( $query->is_home && empty( $query->query_vars['suppress_filters'] ) )
$query->set( 'orderby', 'rand' );
}

?>

The example you've copied from the book is best practice and the function is not "called" before it is defined.

First of all, there's nothing wrong in PHP to call a function "before" it is defined. For instance, this works fine:
myfunc_do_something();
function myfunc_do_something() {
echo "I'm doing stuff";
}
The PHP engine first reads and compile files, then execute them.

In the book example, the custom function boj_randomly_order_blog_posts() is not "called": it's associated with an action hook. Basically, it tells WordPress: "hey, when the event 'pre_get_posts' occurs, please execute my function 'function boj_randomly_order_blog_posts', thanks buddy!".

Note also that if you hook a nonexistant function, or a nonexistant hook, no error occurs. In other words:
add_action( 'pre_get_posts', 'xwkwmklm' );
add_action( 'xklmkxmxklm', 'wxcklwmklm' );
add_action( 'ccwxklm', 'a_real_function_that_exists' );
won't break anything, which is nice to know: hooking to a deprecated hook (removed in a future WP version) or relying on a function that's missing (removed in WP or used by a plugin that got deactivated) won't do the expected action, but it won't break your blog with a fatal error.

Finally, I've said in my reply introduction that this is best practice: in WordPress, be sure not to execute functions directly from the plugin, but always hook them into an action. Refer to "Do not check too early" page 119 for more.