How to insert a <br> for string that I dynamically generated?

In PHP:

ECHO str_replace(': ', ': <br>', 'PHP Reference: Beginner to Intermediate PHP 5');

would be one way to do it.

Your example would replace all ' ' and you'd have all of your words in a single list, one word per row. When doing a replace it is important to make sure you have correctly and uniquely determined your target. Often not a single sting and replace is often that target with its modifications.

Sometimes you replace all the chars in the search string and sometimes you don't.

That's the key to a lot of parsing problems.   And when they get tough there's RegEx, which in and of itself is a special skill.