Emphasizing first letter of string?

Hi, It is getting late, and I do not think I am thinking clearly... What would be the best way to wrap <em></em> tag around the first letter of a string? I know it has to be a combination of str_replace() and substr(), but due to my level of sleepiness I am having a lot of trouble working this one out... Basically, if a config variable == true I want my script to loop through an array of strings and emphasize the first letter of each string. Make sense? Sorry, I am pretty sleepy... Hmmm, I am betting I will discover the answer as soon as I send this email. :D Anyway, any help would be great! Cheers, Micky