If you want to “hook” a function into another function in WordPress, add_action() is what you need. Why do you want to hook a function into another function? Because there is functionality which is not accommodated by current function, so you want to extend it. I use add_action() ALOT in development of Genesis Child theme on WPNest. Since the Genesis Framework itself is loaded by many template hooks, all i need to do when i want to add new element on the child theme is hooking the template hook, instead of making a new template file. It makes me write less code and speeds up the development time once i get used to it.
Enough with the explanation, let’s have an example instead:
Let’s say you want to add this CSS on your <head>, because you want to hide your footer:
<style type="text/css">
#footer {display:none;}
</style>
Instead of coding it directly into your header.php file, let’s do this the beautiful way: make it as a function, then hook it into wp_head() which should be available in <head> section of any good and standardized WordPress theme. These are what you should do, then:
1. Make it as a function
Open the theme’s functions.php file then type this function:
function mytheme_css() {
?>
<style type="text/css">
#footer {display:none;}
</style>
<?php
}
2. Hook the function you’ve just made into wp_head();
Type this function below the function you’ve just made:
add_action("wp_head", "mytheme_css");
That’s it! If your theme has wp_head(); on its section, the wp_head() should output the CSS written in mytheme_css() function.
Well, I hope you find this short tutorial useful. 
WordPress Simple Random Background Using Matt Mullenweg’s randomimage script
16 April 2011
[...] the script for further explanation. Basically what these functions do are printing a stylesheet and hooking it into wp_head() theme [...]
indra.Cakep
02 May 2011
saya coba mengunakan share lagi dong artikel yang baru nya…?