str_replace help

hello all

i’ve pulled this function of the site

function format_html($content){
	$content = "<p>" . str_replace("
", "<br/>", $content) . "";
	$content = "" . str_replace("<br/><br/>", "</p><p>", $content) . "";
	return "" . str_replace("<br/><li>", "<li>", $content) . "";

so when you call the function

<?php echo format_html("$copy</p>"); ?>

it replaces all the

 with <br /> and <p> </p> 

Now i was wondering how could i edit this code so it also accepts my own code such as replacing

<bold> </bold> with <b></b>

I’ve tried editing the above function and for the life of me i have no clue

any help would be great =)