[wickedOne]

though i agree with the comments that there's not really an idea restricted to php (jtheory) and your interests would be good to know (jtheory, dutchbrit), i throw in a wild idea as you didn't provide those…

an application in which you keep track of your fridge's content (throw a cam driven barcode scanner in there) and suggests meals to cook based on the lowest investment (stuff not in your fridge) taking into account the store closest to you (can throw a little location based feature) your favourite meals sorted desc based on a custom rating (client specific rating db?)…

should keep you busy for a while? B-)