On StackOverflow I quite often answer questions by explaining what they should read/who they should read up on, and the names of the technologies: A simple answer might be on the lines of:
"What you are looking for is AJAX. You'll need to store your information in a database – as you mentioned PHP, you most likely will use MySQL. The javascript library jQuery has a really nice way to use AJAX, so I'd start by reading up on …, …, … and …. ."
Rather than:
"Here is the code in 3 files that i've just written without testing anything."
This is a waste of your time for certain questions, though. The ones ending in "urgent please help" just scream that the poster is going to wait 15 minutes, briefly scan the answer sections for code snippets, and move on.
If it's a public, indexed forum, I'd say answering is not a waste of time. Someone can use a search engine and find good answers years after the original question was posted.
That's exactly it – SO has become a list of everyones problems ever, with solutions. I frequently find answers to things I need to know on SO, sometimes years after the person asked.
"What you are looking for is AJAX. You'll need to store your information in a database – as you mentioned PHP, you most likely will use MySQL. The javascript library jQuery has a really nice way to use AJAX, so I'd start by reading up on …, …, … and …. ."
Rather than:
"Here is the code in 3 files that i've just written without testing anything."