On the SitePoint PHP Blog there's a new post today looking at performance in string concatenation where they compare some of the different methods for appending values and which might give you that minute edge.
This could be a more important to your application: large string operations are often handled on the server when generating HTML pages. There are no special libraries in the standard PHP installation and string concatenation is implemented using the dot operator. [...] You can also join an array of strings using the implode function.
In order to simulate a larger set of string concatenations, they create a loop of 30,000 and append a simple string - "String concatenation." - each time. One method uses the standard period append method and the other uses implode after appending to an array. The result of their testing is that the array/implode method takes twice as long as the standard period/append operator. Take into account, however, that for most applications, the performance difference will be so small, it wouldn't be noticed.