I need help figuring out something

I'm trying to write text to an image but some parts of the text should be in italics. For instance, I have the following string. $string = "This is a [i]test[/i] string that [i]has[/i] some parts in [i]italics[/i]" How do I display that on an image using the image functions with php but display the parts between [i][/i] in italics? How would you guy's do it? Thanks, Brent