Since most of my web applications are similar in nature (get user input, validate, write to database, retrieve for edit or page display) I tend to write all the code, run it, and then fix all the errors (which tend to be about the same: strict and database errors).

Having said that, whenever I introduce a new piece of logic (a routine or piece of code I haven't used) I will write a test distilling the code down to the absolute basics. I want to eliminate the less obvious errors before they happen and save the routine errors for that initial run attempt.

Writing code, in some ways, is akin to writing copy for a brochure or a letter. It's sometimes best to just write in broad strokes to keep the spirit of the piece flowing, and then to come back and run the spell checker and make my edits. I.e., don't let the technical detail interrupt that creative flow.