Help with <if condition="$show......... statements

[vb_guru]

How are the <if condition="$show......... statements in a template related to the the php code in the .php pages. For example I want to display some html in a template only if a certain condition is met. I think that I need to use some type of <if condition="$show statement but not sure how to get this to work. Any help woould be appreciated. Does Vbulletin explain this in a manual somewhere. Thanks in advance.